538.com's Nate Silver, looking at one of the Mark Penn E-mails released by Joshua Green of the Atlantic, argues that the "black vote was invisible to Penn." I wouldn't put it that way, but I do think the Clinton campaign made a mistake by assuming that Hillary Clinton would get a sizable share of black votes against Barack Obama.
As the graph that Silver helpfully provides shows, as late as 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.
With the benefit of hindsight, I think the Clinton campaign should have anticipated this. History supports the proposition that black voters tend to vote overwhelmingly for one candidate in Democratic primaries, 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. If memory serves, Jimmy Carter and Bill Clinton also got the lion's share of black votes in primaries in 1976 and 1992. Having attended black political events over the years, I remember how often I would hear 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 seems that in this cycle, black voters, once they saw from the results of the Iowa caucuses that white people would vote for Obama, went en masse for Obama.
Humphrey would surely have won almost all black votes if Robert Kennedy had not run. Similarly, 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—I say in retrospect; I didn't write this at the time—should have anticipated that its black support might vanish suddenly. That meant that its chances of sewing up the nomination by Super Tuesday were much lower than her campaign staffers had expected. And if they had understood that, maybe they wouldn't have spent virtually all their money by February 5.