They're different fetishes all together. The point with blacked being that she only cares about tall aggressive man with a dick, and the nigger literally has nothing else to offer beyond that, hes shit in every other way but that doesn't matter to her and you can't do anything about it. Its entirely masochistic. You're getting off to an honest woman, and she sucks.
Where wanting to have sex with a tall aggressive female whos more "hung" than you is just a part of femdom I think.
I'm not a negrophile so maybe this is just my perspective but sure that's the reason since there isn't overlap with any actual gay porn.
That all said, I like black trannys when its not super obvious that they're just doing it for the money.