Doesn't Blues have a specific form or form type? There is 8 bar blues and 12 bar blues, and they each follow a certain structure in their chord progressions. There is also a Blues scale. Certain notes in the standard scale are flatted or sharped. The theorists could tell us more about that.

If a piece follows a certain structure and scale, then it is a Blues, whether it is "Folk" or Jazz, traditional or commercial.