There is the same proportion of black head coaches as there are black people in the USA population. The rule is extremely archaic, as i seriously doubt anyone is hired/not hired because of race in the NFL. All it does is to force NFL teams to interview one more candidate, so sure... you will get a few surprises every so often. This rule mostly creates the career interviewee, or one who is only selected to be interviewed because he is black... not because he has a shot at the job.