It's worth noting, as Robert George does here, that this notion that Obama's share of the black vote is a problem in a way that, say, George Bush's share of evangelicals weren't. George notes a particularly egregious example, where Bill Schnieder did a whole piece for CNN about the Dems "dependency" on the black vote:
The "problem" with this analysis is -- what's the point? Analysis of the GOP's relative strengths and weaknesses comes down to geographic assessment. Schneider doesn't devote a segment to "What would the Senate look like if white Southerners didn vote for Republicans (which in some states they do to upwards of 70 or 80 percent)?" But blacks voting for Democrats is staged as some sort of "exception" that should implicitly invalidate the reality of the current political situation.
I'd go even further. The best kept secret amongst people like Obama is this--the black vote is the best bargain in politics.
The Democrats monopoly on the black vote is almost wholly based on the perception of the Republican party as racist, and the brand Kennedy built, but LBJ really enacted, in the 60s.
Now, because of the black community's demography, it's likely that Democrats would still get a majority of black votes, even if this weren't the case.