White men like the idea of "big black cock." It's a turn-on for them. Perhaps it is the exotic nature of a big black cock (something they can never have as a white man, considering the idea that black men are considered better hung in general than their white counterparts). Even more interestingly, straight white men like to sexualize black men much more than white men. It's fascinating.
There are indeed big, white boys like the late Billy Glide (hung like a horse). But I believe that black men are preferred for a reason. Now, that's not the say that white men don't like to see big, white cocks. If some white NFL player were to have a 10-incher and make adult entertainment with it, his scenes would be viewed like hotcakes. He'd be the stud of studs.