Without giving it a lot of thought, it seems to me that the blues as music was absorbed into many different black musical expressions, while the blues message as such was no longer palatable to young musicians. This is understandable. Many blues lyrics express a feeling of helplessness, which do not jibe with the civil rights and black nationalist revolt. Except for the occasional artist who drifted into the CP orbit, like Leadbelly, Josh White and Sonny Terry, most blues musicians did not write songs protesting racism. They explored failed love affairs, homesickness, etc.--in fact, the same material that you find in country and western. This should come as no surprise since the social roots are nearly identical. One of the last true blues musicians, Ray Charles, is a master of the C&W genre.
As far as the blues style being assimilated, here are some examples:
1) Jazz, especially the hard bop movement of the late 50s and early 60s. Bobby Timmon's "Moanin'" comes to mind, as does nearly everything that came from the Blue Note label (Freddy Hubbard, Lee Morgan, Wes Montgomery et al).
2) Rhythm and Blues/Motown, etc. Most of Stevie Wonder is very much in the blues idiom, particularly his earliest hits like "Fingertips, part 1 and 2". I remember how wild this song sounded on the juke box at Bard College in 1963, sandwiched in between Jay and the Americans or the Beach Boys. Interestingly enough, another rural sound--gospel--gets absorbed as well, as most of Aretha Franklin demonstrates.
One other angle worth pursuing is the degree to which hip-hop expresses the wisdom from the street that blues once did. I have no doubt that if Robert Johnson came back to life, he'd be a gangster rapper.
Louis Proyect (http://www.panix.com/~lnp3/marxism.html)