To me, the blues are embodied in the musician's approach to the instrument, especially (but not limited to)the guitar and harmonica, or in the case of a singer, the voice. It's a feeling that's magically imparted to the phrasing of the notes or the riff - the perceivable effort put into the way the notes are manipulated to make them "cry," expressively - that engenders the same feeling in the listener, as if the listener now "sympathizes" with the note or phrase. Sometimes it's in the way the musician extends one note, letting that note carry the feeling for what seems like an eternity, driving home the point. Sometimes it's in the intonation.
In this context, the blues aren't restricted to I-IV-V in 8, 12, or 16 bars, amplified or unamplified, fast or slow; nor are they dependent on the words being sung, but more on the sound of the words. Which is why "Sittin' On Top Of The World," with its affirming refrain, is still a blues tune. And why the stuff I consider "uptown" - the slick, refined and polished stuff - like Robert Cray, is blues. Also why technically gifted whiz-bang guitarists, like Eddie Van Halen, or any of the other speed demon "shredders" that readily come to mind, can never play the blues. Long on flash, short on feeling.
I know I'm speaking in abstracts, using terms like "feeling" without defining it. If it grabs me - if a riff or a vocal line makes the hair stand up on the back of my neck - and I have to stop what I'm doing and just listen, then to me it's blues. And a "keeper." No other genre of music has the same effect on me.