I highly recommend Jamelle Bouie's excellent piece on the absence of African American Governors and Senators, an absence that doesn't appear to be changing at all even in the age of Barack Obama. See also his follow-up on black Republicans.
Especially good is his examination of the phenomenon as having little to do with current racist practices and more to do with the consequences of lots of demographic and political factors, some of which are legacies of bigotry and some of which are not.
The one thing that I'd put a little more emphasis on that Bouie does is the active choices that have been available to Democratic party actors, especially in states such as New York, New Jersey, Michigan, Pennsylvania, and others. In these non-Southern states, outside of Illinois, where black candidates have been nominated for the Senate three times in the last couple of decades, Democrats just haven't been nominating anyone (and both Braun and Obama were, at least to some extent, flukish). In other words, it's one thing to say that circumstances explain why there may be fewer good candidates, but that doesn't take the responsibility away from parties to do something about it. Especially parties that rely on African American support to the extent that Democrats do in several states.
The other question I have, however, is to what extent black Democratic leaders in those states -- and again, I'm talking primarily about non-Southern states with large numbers of African Americans -- have pushed hard for these offices. Or have those leaders preferred to work for other goals? I don't have a really good sense of this, other than that the fact I don't have a sense of it may mean that there hasn't been much of a fight over these nominations, at least not in public. I'm very aware, for example, of both efforts to get women nominated and complaints about resistance to those efforts. At the House level, yes, and same with state legislatures, and certainly over city hall in many cities. But not, I don't think, in statewide races. But I'd be interested in reading more about it.