Yes, that’s part of the race play ‘fantasy,’ which is clearly what it is since your potential client was verbally abusive when you declined.

He’s a time wasting, abusive fantasist who should hit your block list.

Just my opinion from my experience.

It's either humiliation race play (slurs etc) or playing out racial stereotypes/power dynamics (boss's wife and servant/slave, types of cuckolding, interracial impregnation fantasies etc). Because I do fetish work I get asked about the latter sometimes (I've found more from asian guys than black tbh) but almost never the former, I would just assume it was someone trying to shock me or see how 'low' they can get a sex worker to sink for money (like when I occasional get asked about bestiality or GBH and stuff... just some edge lord trying to get attention).