As a Mini breeder we totally disagree with this. We saw one "guide horse" several years ago at the Equine Affaire in Ohio. All that poor creature wanted to do was come over and play with my Minis. It took the trainer to get her away, then she walked her owner into the wall. Looks cute, I can see why you'd think it was a great idea, but.... read the website.
Plus, as much as I love and trust my guys, I'd never trust them to lead me around if I were blind. Even my dead-quiet, do anything geldings RUN when startled.