What is blues? The answers depend on the parameters of the moment. I usually think of all music as somewhat derivative so at least I can trace their roots to a more common form. Maybe the blues in its most recognizable form of today comes from the U.S. but I've heard forms of it being championed as African, which is (I think) a little more honest. However it is defined, it's the difference in definitions that make it so much more than we often expect. So long as every definition of it is legitimatized by honest practitioners, the definition can evolve.
Just thinking about defining it is giving me the blues.