I happen to believe that not all cases of "blackface" were meant to demean people of color. How else could you emulate them properly? Al Jolson is an example. If you choose to read the article below, check out where I highlighted a couple of portions to prove my point.
The following is from The History of Minstrelsy.
Perhaps the most popular blackface artist of the 20th century, Al Jolson was a hugely influential entertainer, inspiring future artists such as Judy Garland, Jackie Wilson and Bob Dylan. A natural entertainer with impressive energy, Jolsonís performance style was over the top, wringing all the humor, sentimentality and melodrama from a song. His complete commitment to songs and characters entranced audiences.
Born in Lithuania in 1886, Asa Yoelson immigrated to New York as a child and grew up singing and performing in circuses, vaudeville shows, and burlesques. In 1909, he was hired as a blackface performer in Lew Dockstader's Minstrels, his first experience in the genre. In 1911, he made a smashing debut as an opening act for a play, singing Stephen Foster songs in blackface. His popularity soon overwhelmed Broadway and Jolson became a star and enjoyed a string of hit performances until he retired from the live stage in 1926.
Jolsonís crowning glory came in 1927 in the first full-length talking movie, The Jazz Singer. His semi-autobiographical character struggled to decide whether to pursue a career as a jazz singer or pursue his fatherís vocation as a cantor at a Jewish temple. Jolsonís liberal views on race and his belief that African and Jewish Americans shared experiences of discrimination and hate gave his performances depth and meaning that was missing from most white blackface entertainers before and since. Unlike the minstrel shows that came before, The Jazz Singer called attention to the fact that the performer wore a mask. His performance enthralled audiences around the country. It was even reported that black audiences wept during screenings.
Jolson starred in other major films during the 1930s and beyond, but The Jazz Singer is the film that vaulted him to international stardom. The filmís debut brought a flood of praise and broke all existing box office records. His performance is credited with popularizing African American music more than ever before and creating new interest in contemporary black artists such as Duke Ellington, Louis Armstrong, and Fats Waller. The Jazz Singer also helped make Jolson the highest-paid entertainer of his day.
The practice of blackface continued in the U.S. for several more decades, steadily declining after the 1930s. The Civil Rights movement and changing attitudes on race helped relegate blackface to the dustbin of history."
I'm open to some frank discussion regarding this topic. This is just my point of view right now.