Both Shadowdark and Black Hack add darkness rules. No player races have dark vision or infravision but many monsters do.
Light is an important, valuable, and precious commodity. Both systems encourage the GM to put the light at risk - with environmental effects and enemy actions.
In the dark player characters are at disadvantage for virtually all rolls while the monsters are not.
Maybe my age is showing but I don't see this as adding a lot to the game. Am I missing an opportunity to add something really cool? Or is it just another setting fad?