Onhwy61, the blues stopped evolving as a form in itself because the needs it served became depleted. The rural black country culture that was a product of the Jim Crow south used blues almost like an emotional newscast. When those same people and their children (the generation of WW2) migrated to northern cities the needs changed yet again, but the emotional barometer remained the same. The form of both phases innovated highly emotionally charged ways to express a very specific sociocultural set of feelings and ideas. Those periods (1924 through the depression, and the migratory period after 1945 to 1965) became the basis for all blues of any kind that followed.
You cannot call the morphing into other forms an extension. Yes, the needs changed. R&B, jump, rock n' roll, and blues rock all grew from the 2 models I cited. If you want to say that blues extended itself to become those other forms it is you who is being simplistic. I think categorical distinctions are made for a reason, and those later forms did leave the pure blues behind. The only exception might be the British blues explosion, but in my opinion we can write it away as a wholly derivative venture from the outset.
You cannot call the morphing into other forms an extension. Yes, the needs changed. R&B, jump, rock n' roll, and blues rock all grew from the 2 models I cited. If you want to say that blues extended itself to become those other forms it is you who is being simplistic. I think categorical distinctions are made for a reason, and those later forms did leave the pure blues behind. The only exception might be the British blues explosion, but in my opinion we can write it away as a wholly derivative venture from the outset.