Deep, uselessness. We don't usually think of race and, say, hair color in the same way, but essentially having blonde hair is no different than being ethnically Chinese--they're both just results of genetic adaptations. We're still all the same species...
Races are a funny thing. Supposedly we all came from one race and evolved into the races we know today based on climate and so forth as we migrated around the world... and now we're homogenizing again. The same for "race music" -- as cultures intermingle, stylistic elements fuse together; most of the music I listen to is a combination of eastern European melody, African rhythm, American lyrics, and Asian energy. Of course, I have eclectic tastes. But I suspect that fusion will become more common and popular in the future. In the 21st century, no art or culture or race is isolated from any other.
It was used mostly derogatorily. My parents were livid when they found out I was listening to race radio. (They would have been more upset if they had ever found out I was in Durham watching James Brown and Percy Sledge...)
The name used throughout the South to describe music (soul, beebop, jazz, gospel, rhythm and blues, etc.) produced by African-American musicians. When I was growing up, I used to listen to "race radio"--those stations that played race music from Stax, Motown, Chess, and others. There were also "race theaters"--small venues that booked African-American acts. (I was usually the only white face in those theaters, but I got to see some incredible performances.)