This is fascinating--I didn't know horses were guiding blind people...
I do wonder about their suitability...horses, after all, are near the bottom of the food chain, and share many of the flight reflexes of other prey.
Dogs, on the other hand, are predators, and tend to share the fight reflexes of predatory species.
I can't imagine the horror of being blind and dragged through an unfamiliar area at high speed because my guide horse spooked at something silly. Breakaway harness would essential equipment!
Police horses are trained to be virtually unspookable, but virtual *is* unreal, and I'd think twice before entrusting my entire physical safety to a horse. This from someone who's been regularly riding and caring for horses of all sorts for more than 25 years.