also black-face, 1868 (the phrase itself seems not to have been common in print before 1880s) in reference to a performance style, originated in U.S., where (typically) non-black performers used burnt cork or other theatrical make-up to blacken their faces, from black (adj.) + face (n.). The thing itself is older, from the 1830s.

The old-time black face song-and-dance man has disappeared from the stage. At one time no minstrel or variety company was complete without a team of these stage favorites; and who can ever forget their reception at every performance? It made no difference whether they represented the genteel or the plantation negro, they were always welcome, and as a rule were the big feature of the bill. [William E. "Judge" Horton, "About Stage Folks," 1902]

updated on September 25, 2018

Definitions of blackface from WordNet

blackface (n.)
the makeup (usually burnt cork) used by a performer in order to imitate a Negro;
Etymologies are not definitions. From, not affiliated with etymonline.