Maggie wasn’t blind. She couldn’t walk and used crutches. She pretty much ran the farm with he help of a farmhand. Mickey thought she should see a doctor to see if they could do anything to help her walk again. Bill Horton operated on her and she discovered that Mickey had another wife, Laura. Thinking that Mickey would stay with her if she needed him due to not being able to walk she acted like the operation didn’t work. After Bill told Mickey there was no reason she shouldn’t be walking Mickey brought her the red dancing shoes and told her when she was able to walk he was taking her dancing. That was her assurance he’d stay by her side.
As to why she couldn’t walk I seem to remember her parents being killed in an auto accident and she might have been with them but I’m not positive on this one.