Darkness (and it's precursor, Darkness 15' radius) are some of the most mechanically challenging spells in the system to adjucate. 3.5 sees to it by a dramatic overhaul of the spell.
1st edition:
First Edition PHB wrote:
Darkness, 15' Radius (Alteration)
Level: 2
Range: 1" / level
Duration: 1 turn + 1 round / level
Area of Effect: 1.5" radius globe
Components: V, M
Casting Time: 2 segments
Saving Throw: None
Explanation / Description: This spell causes total, impenetrable darkness in the area of its effect. Infravasion or ultravision are useless. Neither normal nor magical light will work unless a light or continual light spell is used. In the former extent, the darkness spell is negated by the light spell and vice versa. The material components of this spell are a bit of bat fur and either a drop of pitch or a piece of coal.
2nd edition: (spell is located in the second level section to denote it being second level)
Second Edition PHB wrote:
Darkness, 15' Radius (Alteration)
Range: 10 yards / level
Components: V, S, M
Casting Time: 2
Area of Effect: 15-foot radius
Saving Throw: None
This spell causes total, impenetrable darkness in the area of effect. Infravision is useless. Neither normal nor magical light works unless a light or continual light spell is used. In the former event, the darkness spell is negated by the light spell and vice versa. he material components of this spell are a bit of bat fur and either a drop of pitch or a piece of coal.
3rd edition: (note: this is from the SRD as I can't find my 3.0 PHB- it's lacking a material component which I'm pretty sure is in the real book)
3.0 SRD from web wrote:
Evocation [Darkness]
Level: Brd 2, Clr 2, Sor/Wiz 2
Components: V, M/DF
Casting Time: 1 action
Range: Touch
Target: Object touched
Duration: 10 minutes/level (D)
Saving Throw: None
Spell Resistance: No
This spell causes an object to radiate darkness out to a 20-foot radius. Not even creatures that can normally see in the dark can see in an area shrouded in magical darkness. Normal lights do not work, nor do light spells of a lower level. Darkness and the 2nd-level spell daylight cancel each other, leaving whatever light conditions normally prevail in the overlapping areas of the spells. Higher-level light spells are not affected by darkness.
If the spell is cast on a small object that is then placed inside or under a lightproof covering, the spell’s effects are blocked until the covering is removed.
Darkness counters or dispels any light spell of equal or lower level.
3.5 update:
3.5 SRD from d20srd wrote:
Evocation [Darkness]
Level: Brd 2, Clr 2, Sor/Wiz 2
Components: V, M/DF
Casting Time: 1 standard action
Range: Touch
Target: Object touched
Duration: 10 min./level (D)
Saving Throw: None
Spell Resistance: No
This spell causes an object to radiate shadowy illumination out to a 20-foot radius. All creatures in the area gain concealment (20% miss chance). Even creatures that can normally see in such conditions (such as with darkvision or low-light vision) have the miss chance in an area shrouded in magical darkness.
Normal lights (torches, candles, lanterns, and so forth) are incapable of brightening the area, as are light spells of lower level. Higher level light spells are not affected by darkness.
If darkness is cast on a small object that is then placed inside or under a lightproof covering, the spell’s effect is blocked until the covering is removed.
Darkness counters or dispels any light spell of equal or lower spell level.
Arcane Material Component
A bit of bat fur and either a drop of pitch or a piece of coal.
Clerics also have access to a third level spell, Continual Darkness (reversed Continual Light) / Deeper Darkness. It's a 60 foot radius instead of a 15 or 20 foot radius, and it's on equal footing with Continual Light / Daylight for purposes of cancelling or dispelling or whatever, but is' otherwise a very similar spell.
Ok, now to the juicy stuff:
1st edition through 3rd edition all create a sphere that somehow abolishes light. In 3.5 the spell is totally different: not only does it create "shadowy illumination" (presumably meant to mean that everywhere it radiates to, illumination is shadowy, not that it would illuminate a fully dark room in a shadowy fashion) that grants concealment (the 20% kind). A 20% miss chance, similar to 3rd and 3.5 blur, is not out of line for a second level spell. However, unlike the previous versions, it does not create an area you can't see through- you still have line of sight to targets, if one of them is mooning you, you can see that too.
Second edition has a somatic component- the only version to have one. Was it a typo?
Third edition takes the spell from alteration to evocation.
In third edition the darkness apparently emanates from a source and can be blocked by a lightproof container. The source doesn't make it clear what happens if you are *halfway* in a lightproof container, or if you make a pinhole in it- do you get a bullseye unlantern? I'll assume you don't, and that the mechanism is intended to let you "turn off" the darkness emitting coin or whatever it is that can come in and out of a lightproof container.
The versions previous to 3.5 leave some questions.
-Gaze attacks: If you have to see something for it to have an effect, and you can't, it stands to reason it negates these.
-Light based attacks: It would appear that these are negated. Mercury Dragons and Laser manufacturers apparently appealed this for 3.5

-Light based spells: If they are higher level than the spell, these go through in 3.0.
-Heat based attacks: It would appear that these work just fine.
-Heat based detection: In second and first editions, some characters could see heat (and in first some could see ultraviolet). In third they throw this away, giving some characters a kind of sight that shows how things WOULD look, if there was light- the kind of detection this uses is not specified.
-Figments that are visual: If your Silent Image AoE overlaps a Darkness spell and you make an illusion of a soldier that an enemy orc runs through, does he automatically pass his save for that Silent Image by running through an image he could not at all perceive? (The common sense answer is no, it's as if the spell doesn't exist for him- but don't let the rules lawyers on this one) Also note that if someone with True Seeing active were to stumble by, things would be totally awesome.
-Implications for abilities that allow characters to see through magical (but not normal) darkness: True Seeing allows looking through any kind of darkness, implying that true seeing paints an image for you of what something would look like if there was light. By the same logic, the darkness spell has the same implications for any abilities that allow you to see through magical darkness (but not normal darkness).
-Interaction with light spells: You'll look it up each time. You know you will. Under 3.0 (and 3.5), if you have two of these spells of equal level that partially overlap, the overlapping area is the prevailing light conditions. This can be taken to mean that it's the light condition as if the darkness spell did not exist, or as if only the part that is not overlapped still exists. The difference is thus: assume a room 5 feet wide and 60 feet long. At one end is a torch that illuminates the entire room. A darkness spell is cast such that the torch is one one side of it and you are on the other. Even though you aren't in the darkness spell, you can't see anything- the torchlight is totally blocked by the darkness spell. If you cast daylight (the second level arcane version found in 3.0 for purposes of this argument) on an object and then walk forward, you eventually hit an area where the daylight spell and the darkness spell converge. Do you see torchlight in that area, or nothing at all? I would say, nothing at all, because I go with the reading that "prevailing light" means the light you can see from that position, which doesn't include the torch (it's blocked by the still active part of the darkness spell). It also gets confusing if the torchlight, which was supposedly blocked by the spell, is suddenly allowed back into existence on the other side of it.
-Game Balance of Light Descriptor spells. Darkness says that higher level light spells are not affected by darkness in 3.0 and 3.5. There is no sixth edition darkness spell, nor a higher level daylight spell. Is such a spell valid within the intentions of the game? Not always an easy question, it's more frustrating than normal here.
-Game Balance of alternate detection methods: Darkness was probably intended to be a coin with two faces: on the one hand, they can't see you. On the other hand, you can't see them. If you are some hybrid scent based monster, you are probably thrilled at the idea of casting darkness, everywhere if possible. With the increased access the PCs have to these abilities in later versions, NPCs run the risk of seeing great balls of blackness racing towards them, followed by meaty thumping sounds. While it's awesome later as the "Spheres of Annihilation" get their reputation, it's probably not the sort of combat you wanted to model.
-Game Balance of Shit Being Dark: In a closed area, darkness is hard to get away from, and often blocks an entire pathway so that you can't see around it without going in it. Even in an open field at noon, darkness doesn't allow a save and affects an area, preventing line of sight into it, out of it, or through it. That can make a pretty big difference for a second level spell!
-Conservation of energy: while almost all spell effects violate this, this one is one of the more physics problem oriented spells. The conundrum usually starts with "if you have an area that destroys a photon..." and does not end well. The reason for this is because the anomaly stays around for the whole duration of the spell, instead of being a momentary effect. If it blocks infravision, that means it eats up the nice little photons you are always giving and recieving that keep your temperature from dropping. Much worse than someone who knows thermodynamics is someone who knows particle phyiscs. Now you have a player arguing that the spell should fling electrons from nuclei, as without light you can't have charges interect (for worst results, wait for them to realize that protons won't be repulsive to each other). For the four players I've had that this mattered to (shoutout to Alex Silverman, you made me hate this spell first), I've explained that very small interactions aren't messed with, and that small amounts of light being destroyed are converted to molecular heat if such molecules or atoms are available. Hell, doing anything else would be really hard for a wizard, don't you know that? This gives you wiggle room, as you convert from absolute (all photons destroyed) in their mind to a much smoother analog case. Of course, the rules don't care about any of that. If it says it's heat, assume it doesn't get messed with. If it says it's light, it does.
The 3.5 version has none of these issues, because it's not really darkness at all, it's more of a smoky haze-ness. I continue to run the 3.0 version in my games- after all, after being forced through mental gymnastics for years about this spell, the idea of pitching all those thought cycles for a smokebomb effect doesn't appeal to me personally
