I am not a student of this evolution but my guess would be around the time that people of color started communicating how they felt about this. I think that, for many/most fair-minded white people, racism is a hard subject to wrap our heads around because we don't see ourselves. I can speak with a little more authority on the evolution of how gay men are portrayed on stage and I think the arc is similar if a bit behind. How would Jewish people feel if actors had "typical" facial characteristics added for film (or stage)? We have a lot of examining still to do and as I said, few if any of us are without sin.