It may also be internet bias. That is, I'm white, I'm on Reddit and the sites I go to on the internet are quite white heavy. I don't know if you're white, but you're on here, and this place is pretty mayo.

Perhaps it's not that "underground" hip hop is white, but underground hip hop that you hear about through Reddit, the internet, and other white people has a white audience.

Maybe there are far more underground hip hop artists that have black audiences, but we don't ever hear or know about them.