There are many black racers here in southern california. Rashaan Bahati, a race prodigy that races for Saturn, is from Los Angeles.
For him, there was a huge economic hurdle to overcome. Racing is expensive and there is no infrastructure like ones in europe.
B-ball, football, baseball all have a great infrastructure from little leauges to high school sports to college and right into pro ball. They are well advertised, televised and sponsored.
I think for such a small sport in america, cycling is pretty diverse. I see Blacks, Asians and all shades of white in the amateur pelotons.