I think that this book may provide information that could answer the original questions posed by Sam. It is possible that Mantan Moreland, who was seen as funny by all audiences, including blacks "back in the day," could have been shunned as more politically correct times dawned. This is even more interesting in that Monogram included Mr. Moreland in its films in measure to appeal to black audiences.
I just ordered the book ("Mantan Moreland: Mantan the Funnyman") at Amazon.com. It seems that they have the best price ($35.00), and I got free delivery. If anyone else wants to do the same, please hurry, as only "two" copies are still available.