Most of what I see as being advertised as 'Blues Boxes' are usually 12 fret, parlor size instruments. Of course, there are exceptions with a few 14 fret instruments, but just about every 12 fret parlor comes with that tag.
For the most part, I feel this had to do with the re-emergence of the 12 fret guitar, and those ad agencies certainly knew that the modern demographic for these instruments enjoyed playing the blues on them, which certainly represented one of the genres that they were originally intended for, as well as other old-timey types of music. Most of the instruments in the 20's and 30's were small Stellas or Stella like counterparts. These guitars were easily affordable by 'blues' musicians and perhaps for that reason, the 'blues' label stuck.
The electric guitar, when first introduced, was not marketed towards blues players, but rather fell into the hands of the early jazz musicians, and later on, when Les-Paul invented that solid body electric, jazz and later on rock musicians used them. The same is in the case of Leo Fender, whose 'casters' were marketed during the rise of Rock and Roll in the mid-fifties. Of course, many blues musicians who were playing with pick-ups stuck in their acoustics saw the value in those and subsequently started using those.
At least that's my suspicion.
