The Democrats aren't the party of minorities, rather the GOP is the party that comes off being against minorities and thus minorities have no choice but to vote Democrat.
The GOP got 30% of the black vote for POTUS in 1960, then the Civil Rights Act was passed and the GOP hasn't gotten more than 15% of the black vote for POTUS since. Here is a site detailing some history. It wasn't until 1924 that black were even permitted to attend a Democrat POTUS convention, yet by 40 years later they were firmly in the Democrat camp and 50 years beyond that they still are.
http://www.factcheck.org/2008/04/blacks-and-the-democratic-party/