When RJ's records were being primarily marketed to rural black folks, he didn't sell much.

Exactly...these were "race" records for the black market. The whole concept of a "delta bluesman" hadn't been fully developed yet, while the "blues lady" was quite sophisticated (Ma Rainey, Bessie Smith, etc.)...I think that was why Lonnie Johnson sold more then (his sophistication). The record companies weren't out to record "blues"...they wanted to record stuff to make "race" records. Which is why they recorded MJH and Leadbelly doing country stuff as well as blues. That's what was going on in the delta at the time. Anytime either of them wanted to reach into those "blue places" and sing the blues, they could do it with the best of them. I think Leadbelly's Bourgeois Blues among others validates his position as a bluesman...

When Eric Clapton started saying Robert Johnson was godlike, THEN the record-buying public paid attention.

Yep, long after he was gone...

But the Charley Patton, Son House, Robert Johnson influence was perhaps even more directly transferred (i.e., not on recordings, but first hand) to Muddy Waters long before Clapton heard his first RJ.   I think Muddy Waters went on to crystalize the concept of the quintessential "bluesman" and then retrospective looks at RJ got people to think "oh yeah, he was one, too"...One of my favorite delta photos is the one of McKinley Morganfield (aka Muddy Waters) playing a resonator guitar with Son Sims, Charley Patton's fiddler (fiddler?).

The same retrospective look at MJH and Leadbelly causes the comments like "not so much a bluesman as a rural popular singer/folk singer ". I'm not being critical here...I'm just raising the issue that there's some "retrospective labelling" going on that runs the risk of missing what was really going on in Mississippi back then. The concept of a "bluesman" was still being formed, IMHO.

-Greg