I've had blocks my whole life. I'm 31 now. I've had chance encounters with others who stutter but only repitition stutterers and only very mild. I've never met another person who suffers 'blocks'. My dad does but it doesn't really count. I'd always wondered how I would react and I'd imagine it would be much like you. Part of me would want to go up to them and hug them and say "I know what you are going through" but most likely I would want to remove myself from the situation immediately. Seeing someone else would hit too close to home, it's like I'd internalise their sorrow or something. The other person may not even see their stuttering as a negative thing but I can't help it