That's an actual line of dialog from the script. Really.
The Belle Starr story, never actually told in the movies (partially because the real story isn't that interesting..) is told here in early Hollywood color and all the vim and vigor with which they revered the South. The plot hook is that one of the aforementioned "darkies" actually tells the fable as the narrator. Without spoiling the movie, Belle and her husband continue fighting after the War Against Treason, using those traditional Civil War Southern values of robbery, assassination, treason and protecting known criminals to keep Missouri safe for, well,the same people it was safe for before the War. Hey, it works in the movie.
The point made by vitaleralphlouis in his review is well taken. How dare we criticize Hollywood for showing how a loving mammy would help keep Belle safe, or that another "darkie" (their word, not mine) shows Belle's antagonist how disgusting he was. We all know that negroes formerly held as slaves had nothing but love for their former (or in this case present) slaveowners.
This is a classic example of a movie obviously made with care, but looked at today 99% of its viewers would wonder what was in the coffee they served at the story-pitching conference. Because even as a joke, this kind of movie could never be made again, and if there's one good thing you can say about Hollywood, that's it.
Oh, and by the way: a moment of silence for black actors like Louise Beavers who could only find work like this in her era.