To think you have some mystic link into what black people will do over voting for Cain is ignorance, but the fact is if he peeled 5% of the black vote, obummer would lose bigger than moon-dale. If he lost 5 % of the vote and 5% stay home, he does not win in any of the 57 States.
I think Cain can substanially reduce the black vote for Obama. He is still not very likely to get more than 10% of the black vote, but he could reduce the black turnout. What has Obama got to run on - his record? I don't think so. One of the few arrows he has in his quiver is that "whitey" is trying to defeat him because he is black. Even the leftist media is going to have trouble putting that over if the republican nominee is black.
When Obama ran, here in Maryland it was much remarked on that many blacks were voting for the first time because they wanted to finally have a black president. When random black voters were interviewed, they had no idea of what Obama's stands on the issues were - they were voting for a "black president". Now, the only way to get those same voters out again (now that they have finally had their black president) is to convince them that Obama might not get re-elected "because he is black". I think Cain would take the wind out of that sail.
Where I expect Cain would really pick up votes though is from sufferers of "white guilt" - it seems like a sizable portion of republican women voted for Obama and I think a major factor was so they could "prove" they weren't racists. Cain would take that out of play, and they might actually vote on the issues.
However, the continuing homogenous quality of the black vote continues to amaze me. More and more blacks are moving up in the world, moving into professional jobs and opening their own businesses. Why that portion of the black community still votes for democrats in such overwhelming majorities is just beyond my comprehension.