I have a blind Saddlebred that I ride. He went blind at an older age (early teens), but from the same thing. He was already trained to ride, but I essentially had to retrain him anyway once he lost his sight. Her trust level in you is extremely important. Anything you teach her will take twice as long. Verbal cues are your best friend. My horse will relax as long as I keep up consistent light pressure on the reins to constantly direct him, and keep up a steady stream of chatter. If I get quiet, he gets nervous. (I have actually taken to riding him with my mp3 player on, and singing to him whatever happens to play, lol) The main thing to remember is that you ARE her eyes, so if you let her bump things or stumble, she will learn not to trust your judgement. But as long as she is given plenty of time to learn new tasks, and you don't try to push her too far out of her comfort level, there is no reason she can't be a riding horse.
"Keep a leg on each side and your mind in the middle"