I'd say there was a rough style of "art blues" song before _Songbag_ came out that I associate with names like Hoagy Carmichael, Spencer Williams (sometimes among his massive output), W.C. Handy, Willard Robison.

This was recorded about a couple weeks before _Songbag_ came out.
https://www.youtube.com/watch?v=m3SzDoCt7ic

This is earlier and is Robison singing a Handy song.
https://www.youtube.com/watch?v=zhWNbcu6jug