Is there a history of black people using white face to caricaturize white people while owning  or recently owning them as slaves? Nope. And how exactly has the prof's free speech been "discriminated" against? If you want to make an absolute free speech argument then you have to accept a person can be chastised for the things they do. Shouldn't the prof just grow a thicker skin, as some would say, if the reaction bothers them?