I asked my friend about this. He never did it. He says it's something some blind children do at a very young age to kind of stimulate themselves for lack of visual stimulation, but that if parents put a stop to it, as they should, it will not continue into adulthood. Obviously your classmate's parents did not think to tell him that what he was doing inappropriate, probably because Ray Charles and Stevie Wonder did it. Obviously their parents never thought to nip the habit either. That's the trouble; they had the habit and as they were the most visible and well-known blind people for a long time, everybody assumed that it was a normal thing to do.