more common amongst what demographic? in the US? worldwide?

and as for slavery, are we talking about straight up "interracial porn," like those gangbang videos where a bunch of black dudes take on a white chick? or do you mean more literal Uncle Tom / Huck Finn / Beloved cosplay?

Because I think in the US, you're going to find more of a fetish for Big Scary Black Man than German Dominatrix.