I think the Black recruits out of Harlem might have been used to "extract" money out of there in whatever rackets Don Vito had out there.....they were middle men.

Frank might have gotten upset that Blacks were being recruited for things other than pickups.


Another way to look at it though....Don made his own rules and quietly broke at least one rigid rule..he had a non Sicilian consig.(in the book......the other families famously referred to his organization as the Irish gang.....because Tom isn't Italian, so he must be "Irish" hahahah ) So under the Don's authority, maybe Blacks were recruited even though it was not usual procedure. People accept it when done by the Don, but wouldn't accept it otherwise.