"Setting" was commonly used in speech in place of correct "sitting" by both Black and White people with little education and was so written down in many of the collections of folk songs and spirituals. Most collectors tried to copy the dialect and pronunciation of the singer(singers) since this is part of the song's province.
"Settin" was pretty general in 19th c. country America- and is still heard.

The old spiritual "Wish I Was in Heabum Settin' Down" is another example.