giving strength to single creatures isn't part of darkness.
Not yet, anyway... but then, I could have said before Dry Spell that damaging multiple creatures at once wasn't part of water, or said that protecting permanents wasn't part of gravity or aether, until the salvagers.
Actually, I really like this card. Powerful offense, but with a drawback that could make things very difficult for you later. It's also a drawback that prevents too much abuse with something like vampire. (if it were just current hp instead of max hp, for instance, this on a vampire would be free)