In the mid seventies, when I was going to Ohio University, I saw several students in blackface for the annual Halloween orgy. They were mimicking, very crudely, African Americans. (One girl was dressed as Aunt Jemima.) The idea was to offend people. Lots of the students wore offensive costumes. One young man dressed as a prengnant nun. Another had on a trench coat, a pair of sneakers, a pair of socks, and a pair of shorts to which he had attached a large loofah sponge. He was standing in a doorway flashing people. A group of young men wore striped convict uniforms, and chanted, occasionally "We're from----Prison where men are men, women are scarce and sheep are nervous!"
As I said, the idea was to offend people.
Blackface, and the term "darkie", in the U.S. are associated with some rather unfortunate stereotypes.
I understand Harlequin's black mask was, to show that he came from the coal mines of Bergamo, Italy. Just an aside.