At the first look, it seems like a story about tolerance
where three boys get punished for making fun of a dark-skinned child (the coal-tar-raven-black moor
in the german original). As such, the story seems more liberal than the other stories from the book Struwwelpeter
Looking closely, however, the story reveals the morals of the 19th century, where the poor darkie can't help his poor fate of being black, and therefore different from the other children. It is not a Don't make fun of others because of their differences-story, but a Don't make fun of that poor moor, who can't help being black-story.
This is even made clearer when being black is shown as a punishment, a bad thing. The boys making fun of "the moor" are dunked in a barrel of black ink, becoming "moors" themselves, in fact blacker then the original moor, a fact that pleases him immensely.
Concluding: This is not a story to teach children not to be racist, and is revealed to be captured in racist thinking itself, depicting having a different color as a bad thing. A bit of the old white man's burden ideology....