I saw a program on NFL Network. They were talking to a retired coach and he said they don't even consider white guys for CB. Goes down to the college and high school level. They didn't consider Jason Seahorn until the coach they were talkng to saw him play basketball and realized he had the necessary skills.
Blacks are 20% of the population. So there should be 6 black starting QBs in the NFL. I can think of 7. Hotrod, Russell Wilson, Jameis, Cam, Dak Attack, Kizer, and Watson. So actually, they are over represented.