History supports the proposition that black voters tend to vote
overwhelmingly for one candidate, even when that candidate's rival has valid claims on their votes. Case in point: In polls, Robert Kennedy swept the black vote against Hubert Humphrey in 1968, despite Humphrey's long and valiant fight for civil rights laws. Jimmy Carter and Bill Clinton also got the lion's share of black votes in primaries in 1976 and 1992.
Black political events often have speakers calling for "unity." Uniting in support of one candidate is a rational strategy for achieving political leverage for members of a minority group (although it can deprive them of all leverage if that candidate is one no one else will vote for: for example, Jesse Jackson in 1984 and 1988).
It's obvious that in the democratic primaries, black voters, once they saw from the results of the Iowa caucuses that white people would vote for Obama, went en masse for Obama.
In December 2007, Clinton was splitting the black vote evenly with Obama. In early January 2008, Obama was winning a big margin among blacks, and by the end of the month he was for all practical purposes monopolizing the black vote.
Hillary Clinton would have won almost all black votes if Obama had not run. That would have meant that she would have had guaranteed wins in southern states where half or more of Democratic votes would be cast by blacks: South Carolina, Georgia, Alabama, Mississippi, and Louisiana.
The Clinton campaign should have anticipated that its black support might vanish suddenly. If Hillary couldn't compete for black votes, the Mccain/Palin ticket has no chance.
This puts Mccain/Palin at an unfair disadvantge as no other racial group votes this way. Obama only has to win a respectable share of the non-black vote to ensure a victory. Strategy: split the non-black vote. Result: allow black America to select him as our next president.