I would say that areas in LA that became predominantly Black were once pretty White in the old days (south LA, Compton etc.). But there’s always been Mexican neighborhoods all over LA county since the beginning but in small pockets. Nothing like now of course.

Even Boyle Heights and North East LA had a lot of Jews, Irish and Italians in the old days when gangs like White Fence and the Maravilla gangs were barely starting as it was always said White Fence was started to defend the neighborhood against racist whites that were attacking people. The gang East Side Clover was supposedly predominantly Irish until it eventually became all Mexican/Chicano.