Judy Garland in Blackface in the 1938 film Everybody Sing.

Judy Garland performs a song in blackface. In the context of the movie she is not just performing in blackface but actually pretending to be black.  Here is a snippet of dialog:

Ricky: “What’s your name?”
Judy: “Opal Pearl Washington and I’m one of twins. Emerald Ruby passed on but my mammy didn’t mind so much cause we have twelve others, all twins.”

A year later, she would capture the imaginations of children for years to come in the classic film The Wizard of Oz.