I think whether or not the blues has had innovation actually wraps into the way we think about genre. Modern blues music is specifically genre music that calls up the vocabulary and style of a genre that is past it's time - much like equally common occurrence of modern jazz bands playing old swing and bebop standards. The reality is that blues has had a tremendous amount of innovation, but that innovation has become attached to different movements and thus people stop considering it blues. Led Zeppelin, the Grateful Dead, Cream, Jeff Beck, all blues musicians. The vocabulary, the solo-styles, the improvisation, the vocals. All blues. The Black Keys, Baptists, The Sheepdogs are the same.

The blues gets a strange rep because it's just so foundational and familiar in western popular music and music history. This is easiest to see if we go back to the comparison with jazz music. Like I said, tons of jazz musicians play old standards, no innovation, but an homage and dialogue with the roots of their music that is essential in musical practice. If you look at jazz decade by decade as well, you see that it changes wildly, to the point where in the 80's and 90's huge 'jazz' bands were playing world tours with electric instruments, chord structures that had very little to do with old songbook progression, and way less improvisation aside from the presence of a solo section (Return to Forever, Weather Report, Snarky Puppyetc.). In fact, most periods of jazz have had their own name anyway: dixieland, swing, be-bop, hard bop, fusion, etc. Yet, because jazz has always been a bit of a fetishized and outlier genre, it was always called jazz. Indeed, if you're familiar with the discourse around jazz, you'd know that the name is a source of a lot of conversation and debate (what the hell is jazz?) - as early as the 40's, people like Duke Ellington were questioning the use of the title.

So if we relate the jazz history to blues (two branches of the same family tree), you can see that where jazz was always reserved as a term for music from black traditions that favoured the virtuosity of the soloist, the artist and the development of musical forms; blues - as nowaypablo addressed - remained the name of the eldest form of a tradition from which grew american popular music. Modern blues is not innovative because it's not supposed to be innovative, it's period music. It doesn't get to become progressive blues, because rock took up the mantle and took away the name and title of the changing face of blues music.

edit: formatting