It doesn't help for the blind, but I remember my physics teacher (28 years ago) showing us how to tune a single string. He had it stretched over a pair of bridges at least the length of a guitar apart, and he had a 'C' tuning fork. He folded a 'wodge'* of paper over the string, struck the tuning fork, and applied it to the string where it crossed one bridge. Nothing happened!
He repeated this several times, each time adjusting the string tension by use of a tuning nut. As he got close to 'C' the 'wodge' started to be thrown off. This happened with the greatest vigour once the string was correctly tuned.

Nigel

(*Wodge= a piece of paper folded & re-folded to provide a small, heavy, 'v' shaped piece which would sit on the string)