Post 42 mentions what will be a big portion of it, or at least the catalyst, but as post 44 describes, there is clearly something more to the story, it is still unique to blacks.
“All that only makes the Republican-to-Democrat turnaround in the black vote from 1932 to 1936 more surprising, and Ill be interested to see what you find!”
A little thing called the Great Depression.
There was no New Deal in 1932. The blacks were a large part of the poor community but were still largely connected to the Republicans because it was the Republican party who was instrumental in freeing the slaves in the first place. Thats initially where the Republicans gained the following of the blacks. The blacks were now free and many of them were share croppers who had at least gained the rights to work for themselves and see the chance to advance. FDR, because the downturn in the economy because of the crash of 1929 gained power by giving promises that had not been given before. He was actually elected with the Hope and Change attitude, believe it or not. (sound familiar?)
FDR took office in 1933 and launched what was called his First Hundred Days. He had a mostly Democratic congress and pushed through major legislation and issued many executive orders which started programs to produce many government jobs to the unemployed. His programs of regulating Wall Street, banks, and transportation helped many in the black community get jobs. He also started the Social Security system in 1935.
Basically his total package called the New Deal developed a class of people who were dependent on the government for subsistance both through government jobs and social welfare. Because the blacks were a large part of the poor class because they hadnt yet advanced from being slaves to totally self sufficient he gained their support.
The last thing that happened to turn the blacks from Republican support to Democrat (and I think the key that you are looking for) was that in 1937 a deep depression started because of FDRs programs and there was a bipartisan coalition that formed with Republican majority influence. When the War started unemployment dropped and the bipartisan coalition forced the end to many of the relief programs which impacted the blacks to a greater extent then others because they had been the major recipients of the relief program.
Unfortunately, because they began to rely on the relief effort they became dependent on the government. From that point on the Democrats have worked to keep them as a dependent class while the Republicans typically want freedom to advance on ones own.