quote: I don't think a song lyric from 1879 has all that much to do with how black people talked to white people about black people in conversation in the North in 1915.

It shows that race was discussed in that era both casually and frankly and not necessarily in what we would consider polite terms. That song was still popular in 1915, and it shows that "darkie" was one of the more kindly words used, and it uses the term with respect and even affection. If you need more precisely concurrent examples, not necessarily as kindly, there are the "coon songs" or "coon shouts" that were still popular till 1920 and sung by both black and white performers (such as the Dinwiddie Colored Quartet's coon shout "Down on the Old Camp Ground"), and the pickaninny songs that were popular at least from Joplin's 1901 "Pickaninny Days" till Noble Sissle's 1921 "Pickaninny Shoes."

Do you have some evidence that a black person would not have described other black people as "darkies" in a conversation with a white person in 1915? I wouldn't be surprised if a black person today were to tell me that jazz was played originally by blacks in New Orleans, though 50 years from now "black" will probably have become a racist and insulting term. And in fact, about 25 years ago in Kansas City a black man I had just met told me the reason he doesn't like blues is because it's "nigger hillbilly music."