Umm, "black" and "brown" girls are not the same thing, and even calling someone "brown" is so fucking offensive I want to cyber перфоратор, удар, пунш Ты for your racism. A person who is "black" is usually of African descent and the only reason calling them "black" is okay is because there's no negative connotations behind it. Calling someone "brown" is distasteful since it's almost always used as a slur against Middle Easterners. Ты need to educate yourself, because racial ignorance is not cute.
But as far as the original Вопрос goes, Harry has mentioned several times that he has a crush on Rihanna, so obviously if he's attracted to a black celebrity he would дата one too. No idea about Niall.
I think harry styles should дата a black girl because if all he dates is White girls then that would be offensive cause I'm black and I have a humongous crush on him and would Любовь to дата him but im only 113 but I'm still waiting!!!!!