I don't know if there can ever be a seminal moment, but you can pinpoint the seminal beginning and that would be with the Brown vs BOE and the subsequent desegregation of America. I think once people intermixed in society as "equals" (at that time, of course, things were not equal), a lot of the learned or inherited ignorance that prevailed in the "blacks are subhuman/subspecies and whites are a superior biological race" melted away under the light of lived experiences and realization that biology is more or less equal opportunity employers for our capabilities, features and attributes in both physical and mental.
Satchel let me be clear here for you, because you seem to be trying to stump-the-chump when you really don't grasp my position: I haven't published a paper in the Mid-Atlantic Journal of Social Sciences for a Modern World under the title "Classism as the new modality for racism in America" so I'm not going to have a bunch of scholarship at my disposal and and your beck-and-call, but you asking a bunch of mind-numbing questions because you don't grasp what I'm saying isn't disproving or discrediting my opinion. It would be more productive if you could push back or challenge with your own take, in agreement or disagreement.