That phrase is usually a reference to opioids and getting hooked but it improving your music.

The lyrics of one of his songs has him begging God to specifically save him.

If he did sell his soul, he was asking God to save him from the deal he made.

If his is the kind of music that results from selling one's soul to the devil, I'm all for it.

and his reputation as a man who sold his soul to the devil.

Has anyone ever read or heard of this topic?

I am fascinated by his story.

If you know of him, his stories, and his music, what do you think?

If you haven’t heard of him, and are interested, check out his wiki:

https://en.m.wikipedia.org/wiki/Robert_Johnson

(Note: lots of bluesman are quoted as saying that they sold their souls for musical mastery, and lots and lots of scholars have researched and written books about them and the topic).

Robert Johnson is considered the preeminent and most important man, the foundational figure of Mississippi Delta Blues. His limited yet groundbreaking music is on itunes, if you are interested in listening.