Sounds weird when you ask the question that way, but if you ask "is there a non-black marble in the bag?" the answer is clearly no. So the question is more like

Sounds weird when you ask the question that way, but if you ask "is there a non-black marble in the bag?" the answer is clearly no. So the question is more like: Should "all marbles in the bag are black" be exactly equal to "not(is there a non-black marble in the bag)".