Orson Welles and Laurence Olivier both played Othello in black-face. I have no objection to black-face when it's relevant to the plot. It might have seemed unfair at a time when black actors weren't cast in white roles, but we've gone far beyond that now. It's quite difficult to find a straightforward, realistic production of Shakespeare. There was a row about this at the Globe a few years ago. As a former English teacher, I've sat through no end of modern-dress productions, with punk witches and actors in SS uniforms. I'm glad I've seen films of great actors like Olivier and Welles playing the role. Just as I am that Glenda Jackson is able to give her interpretation of Lear, or Adrian Lestor (?) can be a black Hamlet.
I don't think there should be a blanket ban on black-face. I think we should aim to be mature enough, as a society, not to need one. It's obvious when it's done to mock and cause offence. Leave Morris dancers alone. (There's lots on this on Google.)
Someone said that the treatment of Native Americans should be taught in schools. My dgd has just done a GCSE History unit on the American West. I recommended an excellent series of detective fiction set on the Navajo Reservation by Tony Hillerman. He's white, but has been awarded the title of Honorary Navajo. It's a gentle introduction to the subject before moving on to classics such as Bury My Heart at Wounded Knee, etc.
Btw, when you watch old Westerns, the Native Americans are usually Navajo. (Their reservation is close to Hollywood - it's huge, the size of Wales - and, as a tribe, they are more adventurous and outgoing than some. Read up about the Navajo Code Talkers and the part they played in WW2.) When they're talking Navajo, they're not going 'rhubarb, rhubarb'. They're making rude remarks about the white actors.