A definition of the blues? I'm not going to attempt that one. However, I can respond to the next part of your question, about Eric Clapton, et al.
When I was in my early twenties, and first discovered the blues, I learned the old acoustic country blues from the 1920's and 30's. At that time, I considered anything electric or variant from the "standard" patterns to be a bastard child of the blues: e.g. Keb Mo, Stevie Ray Vaughn, Fabulous Thunderbirds, even Muddy Waters (did I really believe that?).
Through the past ten years (I'll be thirty in a couple of months; Yike!), my tastes have changed, and I have progressed through the progression of the blues. I've developed a taste for the electric blues of the '50's, the British blues of the '60's, and even a bit of the psychedelic blues of the '70's.A lot of purists will say that the majority of the style of non-traditional blues played in the 1970's, 80's and 90's isn't the blues at all. However, as has been pointed out in other threads, musical styles change and progress. Clapton, Vaughn and others are playing the blues of the 90's. I don't really like their stuff; I'll always have a soft spot for most of the stuff made from the 20's through the 50's. But they are the natural next extension of the blues. Big Bill Broonzy and the wide assortment of dead Johnsons would probably be playing just like that if they were alive today.