If you're really interested, it's several things. If you're a women, black cocks are generally bigger. Black men can usually last longer, and don't forget the taboo interracial aspect. Given a choice, the black men I know will forgo the use of a condom, which hightens the danger, although it also highens the pleasure. Black men tend to be more dominant, which suits me. There are other reasons of course. 

If you're a man, well, thats why women that know, go black.