The big difference between Cain and Obama and one that Cain may not be able to completely overcome with the black voter, and that is the approach to entitlements/government intervention.
Cain expects blacks to be responsible and accountable. I'm not sure that will go over well with many blacks who are all too comfortable with government handing out free shit.
Cain is certainly a threat to Obama and may, in fact, entice a fair number of black voters. But in the end, most blacks will vote Dem, just as they always have.