I don't think it worked that way, The African-Americans had their own connections. They did use Italians at times, but they had multiple sources. There were mob guys selling to them on occasion, but also White guys who were not part of the mafia. Plus they were right next to Spanish-Harlem, which was there biggest source. That leaves out their own people, who bring drugs in from the south. Mexico, then over the border, then Harlem.
That last one shows how some things never change.