Hoover won 2/3rds of the black vote in 1932. Blacks weren't especially put off by anti-Catholicism (in fact, they probably had a lower view of Catholics than WASPs). It's also not like the Democrats had blacks in their Southern party organizations either.
It's not the anti-Catholicism I'm going off of (Catholics and African Americans were often on opposite political sides due to a deeply held animosity between the groups that dated back to the Civil War era), but the fact that Hoover encouraged stripping black Republicans of patronage in favor of the businessmen who made up the elite of the Republican Party.
Pretty much, Hoover gave them the finger in the hopes of appeasing Southern White Protestants.
Which is why I'm surprised he won the vote in 1932.
Most probably were not aware of Hoover's actions and just voted with the party of Lincoln like they had been for 70 years. Most people didn't follow politics that closely and even today most people don't keep track of appointments unless a big story is made out of them. At that point, people had far more pressing concerns to worry about.
