How about a boost against Dark Types? such as +50% light boost on moves? being flashed after getting used to darkness sure could hurt the eyes.

As well, perhaps Flash could remove Darkness for 3 or so turns?

How about a Speed Boost, instead of evasion boost, for Dark Types? Speed is a much more well defined stat than evasiveness, and dark types are accustomed to dark areas, and thus should have better chances of navigating them.

Darkness could also hamper Chances of catching a pokemon, unless you are using a dusk ball? Should be hard to throw a ball when you can't see. Perhaps there could be night-vision goggles that allow the trainer to catch pokemon in dark areas, or perhaps it could be a hold item to boost accuracy in dark areas?