I must disagree with you. To say that B.B. King is overrated as a blues man is like saying Lou Gehrig was overrated as a ballplayer. Gehrig wasn't the greatest of all time, but he was among the best players of all time. Same with King. BB is just one of a handful of blues artists (alongside Muddy Waters, Elmore James, Albert King, Howlin Wolf, to name a few ) who were there in the beginning when electric blues came into being after being born in Mississippi, raised in Memphis, and then settling in Chicago.When blues spawned R&B and Rock N Roll, the older generation of bluesmen began to fade away. Some died, some had local notoriety, some had moderate success. In the case of BB King, his "popularity" if you will, is due to the fact that he continued to make records and perform continuously throughout the past 5 decades. What set him apart from the others (and this is where I can see your point somewhat) is that he "went Vegas" - literally and figuratively. So, in essence, he basically was the only "name" blues guy that was still around and therefore when someone asks one of those wannabe blues fans who they like, BB King is always a safe choice since most everyone knows his name and that he's a blues artist.
As far as Clapton goes, he is simply a Rock guitarist who is steeped in the blues - influenced heavily by King and all the others. Not to say that Eric isn't a great blues player but blues is just one of his styles. Stevie Ray Vaughn was more true to the blues than Clapton, but even Stevie can never be classified as a "real" bluesman. The only "true" bluesmen were black men raised in the southern US during the 1920's, 30's, and 40's. Their music was derived from and based upon hard living, hard times, oppression, and poverty. Blues is, in my opinion, the only original form of American music.