We (as in, the majority of blacks in America) vote Democrat because it's something we just do. Our parents are democrats, so when we reach voting age, we vote democrat too. We vote democrat also because the GOP is the anti-thesis to a lot of problems blacks face (ie crippling social programs). Not to say that the Democrats are really that much better, but they're still better in that regard. The GOP give no flying fuck about poor people, and an overwhelmingly amount of blacks are poor. That is why we vote Democrat. Either you are poor, or you have family that is poor (which is every black American, rich, middle, or poor).
The thing about the black vote is that many are socially liberal on programs, but tend to sway towards conservatism on actual issues like gay rights, mostly due to intense religiosity.
There are reasons for voting for Democats, but most of them are legacy reasons, not modern reasons. Of course, they may be able to swing some black conservatives with more exposure to the likes of Herman Cain.