There's a prevailing belief (not that I myself believe it, I'm just the messenger) that black Johns are more violent. If you're running a scam, you'd want to do so at the lowest risk of your safety.
Some legit providers do exclude black gents though, so this alone isn't a 100% guarantee of a scam. I myself try to avoid them altogether, both due to the scam risk and due to the ethics of not servicing a particular race.