The performance of blackface by white people is always a reference to and a re-enactment of the demeaning tradition of blackface. The law professor was a white person impersonating a black person--it's taboo BECAUSE of the white tradition of blackface minstrelsy. If you don't understand this it's because you don't know enough about the history of anti-black racism in this country. 
"White blackface performers in the past used burnt cork and later greasepaint or shoe polish to blacken their skin and exaggerate their lips, often wearing woolly wigs, gloves, tailcoats, or ragged clothes to complete the transformation. Stereotypes embodied in the stock characters of blackface minstrels played a significant role in cementing and proliferating racist images, attitudes, and perceptions worldwide." 
https://en.wikipedia.org/wiki/Cultural_appropriation