I do believe it was the bluesman Blind Blake who played a 9-string. I can personally vouch for Big Joe Williams (the country blues singer---not the Basie Orchestra vocalist) having played a 9-string. When I was young in Chicago Big Joe was making some records for Bob Koester's Delmark Records label. I used to catch every gig of his I could. He always played a 9-string guitar. His was (at that time) a 6-string guitar to which he had added a 3-tuner geared metal strip to the top (the end) of the tuner wood stock. That really didn't fit there. As a result, one of his tuners was outside the width of the tuner stock. (Hope that makes some sense to ya. I can visualize it but am not sure if my words paint the right picture for you.) Still, the string running from that tuner was held in it's proper place by the groove in the bone nut which was a bit deeper that normal in order to accomplish that.

Now, when I decided to change my 6-string D-76 Martin Bicentennial guitar into a 9-string, it was because my hands had been going numb for several years. I was told I had carpal tunnel syndrome. I was looking for any way I could to make picking my instruments easier. I thought that getting more sound when strumming might help me since picking intricate stuff was fast becoming a thing of the past for me. What I didn't count on was that my left hand technique suffered because the added strings made chording more difficult. Still, since I'd changed my Martin, I figured I would stick with the 9-strings. That I did until I could barely play the big guitar. Then I bought a 000-18 and went back to 6 strings and actually tuned down a full note and used a capo to lower the action which was actually already very low.
By the way, to add the 3 extra strings, I installed 3 straight-through banjo planetary gears to the center of the tuning stock. 3 small drilled holes accomplished that pretty easily. The ball ends of the strings went into the six bridge-pin holes----2 strings to a hole. A new bone with slight guide grooves had to be installed as the bridge saddle. I doubled the 3 treble strings and left the 3 bass strings single. That eliminated the muddy sound you get from a normal 12-string guitar with the octave stings in the various pairs. The treble strings were fuller sounding when strummed. The bass strings were clear and easy to hear like on a normal 6-string guitar.

Art Thieme