In California, I tested into the gifted program after the first grade. This was in 1971, a few years before Prop. 13 was passed, which cut taxes and allowed my parents to keep their house, but destroyed public education in California, IMO.
My mentally gifted minor classes in elementary school were fairly diverse. You had black, white, Latino and Asian kids. Among other things, we had television production classes. During junior high (1976-79) and senior high (1979-82), things changed markedly from a racial standpoint.
I was a voluntary busing participant, as were other black and Latino students, but there were virtually no black kids participating in gifted education. Our honors English program had only three black kids, period, and this was a decent-size high school.
As for reasons why black kids may not be in gifted programs, other posters have hit on reasons in this thread and the teacher-related thread, particularly corrosive anti-intellectual and anti-education peer pressure.
Kids learn a lot from others and the media, and it certainly doesn't help when a smart black kid is being portrayed as a dork on TV.