Who was making the films in Hollywood at the time? Wasn’t Hollywood a bed of liberals and socialists then as it is now? The champions of the downtrodden? So if thats the case who was doing the stereo typing and exclusion of inclusion by blacks into main stream Hollywood?
Actually most of the studio heads then were fairly conservative and mostly Republicans.
Sometime after W.W. II, certainly by the 1950’s, there was a change as the old guard died off.