The funny thing to me is, with every other superhero, they have to put on a black costume whenever it's supposed to show that they're "going dark." Batman is supposed to already be dark, so why shouldn't he have a black costume by default?
As for this claim that gray is stealthier than black-- first of all, Batman's suit in Nolan's movies IS gray. The way it is lit makes it look black, but it's actually matte gray. I see no reason to make the cowl darker and put panties on it when there would be no discernable difference in most lighting conditions anyway. Second of all, if you're going to use that argument, then surely you would not object to Batman's suit having an urban camo pattern, since whatever benefit a middle gray suit would have would be minuscule compared to actual camo?
There's your perfect Batman suit material. And you wouldn't be able to complain about it, since your argument is that black isn't stealthy enough.