I think it was the Gallos or Gambinos honestly.

Black guys were affiliated with the Gallo crew.

Also a convenient scapegoat since it'd point away from the Mob on the surface.

Also it was a suicide mission and better to send an unmade associate than a crew member (plus, again, the need to point away from things).

The Mob believed it enough to whack Joey Gallo out over it, so I think if they believed it, it's probably that his people were responsible. The Mob are gossips and word spreads. They wouldn't have whacked him out if they didn't really believe it was his crew.