As someone who's been around blind people for a large part of his life, it's not that hard to show that someone's blind without them looking the wrong way (which would be ridiculous, they still have ears) - blind people often develop minor tics like nodding their head or move in other slightly "off" ways (think Ray Charles). This wouldn't be hard to combine with some crochety old man movements as well - and elderly, blind, crusading Dandalo is definitely the most interesting age for him to be portrayed, imo.