Now, there's an interesting point that Johnny Shines once made to me in conversation. He said, If you sing, my baby's pretty, she got (whatever attributes you think are fine), that ain't the blues. Blue for Johnny Shines was the unburdening of trouble in lyrics.
So for him, one of the great storytellers of the blues, blues was not defined by form or scale but by lyrics. For him, a huge chunk of what gets called blues he would consider not to be blues.