There's a distinction between rock musicians who acknowledge their blues influences occasionally, and those musicians who strictly play blues, however one cares to define the genre.

Regardless, just because Leadbelly sang "Bluetail Fly" doesn't diminish his stature as a bluesman, no more than Hendrix singing "Purple Haze" diminishes his stellar reindition of "Red House." One listen to Hendrix's version of that song should be enough to convince anyone that he knew blues inside out and upside down (literally). Perhaps the high decibel trend of those early so-called "psychedelic" times stems less from the desire to do sonic damage and more from the fact that those '60's Marshall heads had to be cranked to get the siren-esque tone of mournful urgency those guys loved so much. And while Leadbelly's "field hollers" are somewhat more subdued than an out-and-out primal scream, they both serve to reflect the same feeling of frustration born of despair.

Even Clapton describes himself, when all is said and done, as a blues guitarist, notwithstanding his success with the 'unplugged' ballads. And he, of all people, should be the foremost authority on his playing. Those rare occasions when he is caught on video noodling around on his Strat seem to confirm this. It's those same five notes of the pentatonic scale, done to death but never losing their ability to evoke the feelings we associate with the blues.