It's got a lot to do with racial tensions and emasculation. I think that because black men have larger penises it makes things tense. Feelings of inadequacy and extreme jealousy become sexual enhancers. The primal feeling is like: the other tribe is conquering your women and you have been defeated. another reason I think it's so hot for white guys, I think we vicariously fuck the women through the big black cock. Like we get to imagine fucking her with a giant dick.