Probably happens in real life, but like others said - extremely rare. There's an increasing amount of interracial sex happening between white men and black women especially on college campuses, but still well below that of white women and black men. I've also heard of some black women into being dominated by white men sexually in a sort of BDSM or race play (I'm talking sexual fantasy play only here).
I don't think black cuck/white bull will ever be that common because the sexual performance and size stereotypes are applied to black men, not other men. Also, throughout a lot of history the focus has mainly been the "light" female and "dark" male pairing which is what makes this lifestyle so appealing to some because the contrast is most obvious.