Agreed on darkness. The cleanest way to make it work is to treat it as a zone where light is simply negated--whether that light is being generated inside the zone, or entering from outside, doesn't matter. So, from the outside, it looks like a sphere of absolute black*, and from the inside, you can't see at all. Darkvision is blocked because... well, tell me how you think darkvision works normally and I'll come up with a reason why it doesn't work here. 
Anything else gets into a hairy tangle of figuring out what you can and can't see under what circumstances.
In a more general sense, it would be interesting to distinguish between "ambient light too dim for you to make out anything" (a dark night, a cave beyond the radius of your torch) and "no light at all" (a cave when your torch has gone out). It would be essentially a fourth category of illumination, in between dim light and darkness. But I'm not sure it's worth the cognitive overhead. The current system seems to get the job done pretty well... at least now that they've errataed the original rule that said creatures in darkness were blinded.
*You get mad props as a player if you can figure out a way to bluff enemies into thinking your darkness spell is a giant sphere of annihilation.

Anything else gets into a hairy tangle of figuring out what you can and can't see under what circumstances.
In a more general sense, it would be interesting to distinguish between "ambient light too dim for you to make out anything" (a dark night, a cave beyond the radius of your torch) and "no light at all" (a cave when your torch has gone out). It would be essentially a fourth category of illumination, in between dim light and darkness. But I'm not sure it's worth the cognitive overhead. The current system seems to get the job done pretty well... at least now that they've errataed the original rule that said creatures in darkness were blinded.
*You get mad props as a player if you can figure out a way to bluff enemies into thinking your darkness spell is a giant sphere of annihilation.
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