What is the history of blackface outside of America?

It was certainly a cultural phenomenon in Cuba (also a territory with a large ex slave population). "Teatro Bufo" (roughly analogous to "buffoon theater") would often use blackface in its mix of music, comedy, and social satire. Classic character archetypes would include "El Gallego" (the Galician, who was a stand in for all recently arrived Spanish immigrants), "El Negro" (the Black, who was often a white actor in blackface) and "El Criollo" (the native born white Cuban).

Plays would commonly be some form of social satire which would be told through the interaction of these three archetypes. This was an important art form in Cuban popular culture during the late 19th century and throughout the first half of the 20th, because it allowed people to make political statements without fear of repression. Sort of like how Voltaire´s tongue in cheek criticisms of society and particularly of the Church would be placed in the mouths of his own fictional characters in his novels.

In the reproductions of bufo theater I have seen, the blackface character (who could also be an actual black man at times) would basically play something akin to a cross between trickster Brer Rabbit and the childlike slaves and ex-slaves in Gone with the Wind.

While not exactly Teatro Bufo, you can get an idea of what it was like if you look up the recent film El Ojo del Canario (The Eye of the Canary), about the life of José Martí. One of the pivotal moments of his life is when one bufo-like play was disrupted by Spaniards who didn´t like its message, leading to an infamous massacre of theater goers.

Edit: https://youtu.be/ff8amV9VWCo?t=3798

While racism in Cuba is still certainly an issue, bufo became less popular as the 20th century dragged on and it seems to have died out during or before the Cuban revolution. Its racist overtones, and the biting wit of its political satire, probably helped lead to its disappearance under the revolution.

Edit: While I studied it in Cuba with Cuban sources, film, slides, etc., it seems there is a book about it available in the US for those interested: Blackface Cuba, 1840-1895 by Jill Lane.