My two cents as just one person who reads a lot of 19th century writing and song texts:

"negro" was (past tense) an acceptable and neutral term for a Black person. It was used in polite speech. It sounds too formal in a "folk" song.

"nigger" was deemed uncouth by high-status speakers. However, it was part of vernacular speech. Sometimes it gave offense, sometimes it didn't. Depends on who was saying it and their intent. Black people used it as often as White people. In song, often it is a marker that the song was sung by a Black person (who spoke the vernacular) or else was a song meant to represent what a Black person might say (i.e. to put the song in an authentic "Black voice").

"darkey," even if not intending to give offense, marks a racist. Black people did not use it. White people who thought they were too classy to say "nigger" used it.

There are probably many exceptions, of course.

While "nigger" is obviously the most dangerous word today, I think there is some room for considering it *in historical context*, especially since its erasure can actually end up erasing Black voices and "vernacular" (and not intentional racist) White voices from history.