Fits with my personal experiences. I never really thought about it before, but after one particularly excruciating hour-long wait during the dog days of summer, when the post office AC was out, I started monitoring the race of the people who served me at post offices back before they consolidated the lines the way banks do. And the slowest lines were always manned by blacks.
I think that is an answer in search of data to support it.
That's not my experience here at the plant where I work. Our black employees are as good as anyone else. But we're getting the cream of the crop because ours have either (a)taken themselves out of the inner city completely or (b)are commuting a long distance therefrom.
IOW, they've rejected typical inner city values.