Just "Dogs are black" ought to be accepted. While it isn't a true statement, it is grammatically correct, right?
Edit: I would understand if it were just "kutta kala hai." because "dog is black" doesn't make sense, but "dogs are black" does.
A better translation for "dogs are black" would be "कुत्ते काले होते हैं।" Since you are implying that all dogs are black and that its color is an essential quality, you would use होता/होती/होते.