Some cities exempt guide dog owners and other service dog owners from scooping laws, but most do not. There is no need for an exemption because blind people are just as capable as sighted people in doing most things, including cleaning up after their dogs.
A person doesn't have to see poop to pick it up. Like anyone else, a blind person knows which end of the dog is which, and the dog only toilets on command so they know the when and where of poopology. A hand is inserted in a plastic poop bag like a glove and the hand is run down the hind leg of the dog to the dump zone. Then the scooper feels around for the warm squooshy stuff, grasps it, and turns the bag inside out. It's exactly the same for a sighted person who walks their dog at night and must scoop in the dark.