Hmmm...interesting question.
There are places where The Blues crosses over with jazz and rock, as well as with folk. For instance, there are Led Zeppelin and Cream songs that could be considered blues by some. Covers spark still more questions...Is Cream's cover of "Crossroads" still blues, or is it rock?
I have a hard time trying to define the blues. For me, it seems to have more to do with the feeling that inspired the music, and the feeling it leaves the listener with than with any specifics.