Thanks everyone for your opinions. I've heard stories about blind horses who went blind later in life and were ridden fine, but I've also heard stories about some who weren't ridable anymore. I've never heard anything about anyone breaking a blind horse, however.
Something the vet mentioned to me was a cataract removal surgery, that would return her eyesight to her. It's very expensive, and the vet didn't seem to think it was something that has a great success rate. Has anyone ever had an experience with a horse who has undergone this surgery?