The question is this: who's applying for those positions? Just because one is black doesn't entitle a person to quotas for their position.
It's probably the blacks' lack of interest in the GOP that is prompting most of them not to apply for jobs with those staffers, not the other way around. Does your typical inner-city black man look at Congress and say they vote Dem because there aren't enough black staffers in the Republican offices? Most of the time, the Republicans represent areas that don't even have that many black people in the first place.
No, the inner cities are teeming with public government services, on which the residents become dependent. Because black population is so heavily concentrated in those areas, that's why they vote Democrat so much. City folk see government and think it's great. Rural folk see it fail so miserably because of lack of scale and distrust it.