Hi Jeanette.. and welcome to the forum
I'm guessing that this (lapse of being potty trained) might just be a temporary problem with your pup, since his surgery was fairly recent. Between the adjusting to blindness, and probably pain meds he might just be suffering a bit of confusion right now. When our dog first went blind she was having some accidents in the house (which she'd never done before) but started going outside again within a week or so. Did your dog use a doggy door to go out before, or how did he signal you that he needed to go out (bark, turning, looking at you etc.?) If you think that the blindness is making it harder for him to let you know you could always try training him to ring a bell hanging on a ribbon by the door? Please let us know how things go.. and I hope this will be just a very short setback for you!