Doug, my friend, the democrats didn't just "promise" the blacks, they delivered. Some of the things were the Civil rights act which Lott fought against. Integrating the armed forces was another. PACs etc. Voting rights, well, except Florida.
Being on the bottom of the social heap for so many years, they know that their support comes from the left. They also know this: A POOR MAN VOTING FOR A REPUBLICAN IS LIKE A CHICKEN VOTING FOR COLONEL SANDERS.