Not to take anything away from the great, and deservedly legendary, Robert Johnson, but... he is by no means the only blues artists whose recordings were mistakenly presumed to feature two guitars.

I've heard that the same comment had been made about Mississippi John Hurt, and it would make perfect sense for the same misperception to have occurred when first listening to the recordings of any of that first generation of blues players to have cur records.

When we think about "defining" the blues, the first features that come to mind are the flatted third, the 12-bar streucture, and the emotional lyrical content. But, from the standpoint of guitar technique, the blues convention of playing a steady pulsating rhythm with the thumb while playing a relatively independent melody line with the fingers ~ that's something unique to this musical form, or at least it was unique and pretty much unknown elsewhere until a later generation of enthusiasts learned to play like the blues masters, and later eventually applied similar techniques to a wider variety of music.