Which county has the highest mixed race population?

[Stranger in a strange land]

It would be interesting to see the county with the highest white-black mixed race %.

I would say the counties along the "Black Belt" from Mississippi to North Carolina would have highest rates of black-white mix.

Doubt it. Mississippi has one of the lowest percentages of mixed-race people, and there's still widespread disapproval of black-white dating and marriage throughout much of the South. Things have changed a lot in the last 40 years, but they haven't changed that much.