-White husband suspects wife is having an affair. Husband is determined to confront the other man if it's true. Hubby comes home early and catches them in the act and then confronts big, powerful black dude. Black dude punks out white guy and gives him the option of getting his ass beat and further humiliated in front of wife, or watching the action (or getting throat gagged if you want it to be bi.)
-White boyfriend owes money to the mafia. The mafia is black. Secretly made a deal with them to use wife as collateral in case of default. White guy, feeling particularly horny, decides not to pay them even though he has the money so that they can come and collect the collateral. Boyfriend pays them afterwards as long as they agreed to do it again. Would be best done with a prudish, upper class looking milf who acts as though she's racist.
-Very simple and fairly vanilla but I think still hot- White couple hangs out with a black couple, have drinks and decide to swap. Black male and female are dominant and white couple is submissive.
-Young white couple are drinking at a bar. Black guy comes up and disrespects white girl. White girl expects boyfriend to stand up for her. They go out to the parking lot and white boy gets manhandled by young black guy, gets knocked out, comes to and looks up to see his girlfriend sucking the biggest cock he's ever seen. Black guy taunts white guy saying things like, "see white boy, that's what you get thinking you're tough." It can go in many directions from there.