Not technically before 1950, but this 1951 recording of Lone Town Blues by Junior Brooks sounded years ahead of its time. FWIW, Brooks’ vocal style was obviously influenced by the aforementioned Son House.
Regarding the aforementioned Charley Patton: High Water Everywhere Part 1 is one of his masterpieces. That’s one guitar and one voice. Patton may be my all-time favorite performer, period.
From the RnR ballad school, I’ve always been a fan of The Ink Spots. Listening to their approach to slow songs you can hear how that early Rock and Roll (50s and 60s) brought their groove to the house.
ETA Leadbelly. Holy crap could he bring that crazy evil sound to his songs. You might know one of his from Nirvana. Where Did You Sleep Last Night?
Or just for fun: Leadbelly, Mr. Hitler.
House of Blue Lights by Ella Mae Morse (1945):
Wynonie Harris Good Rockin’ Tonight (1948):
Papa Charlie Jackson Shake That Thing (1925):
Charlie Christian and Benny Goodman Boy Meets Goy (1940?):
Milt Brown and His Brownies Keep-a-Knockin’ (1936)