Horses can and do suffer from vision loss. This can be bilateral or unilateral.
A blind horse is not completely useless. You can certainly ride a blind horse in an arena or on all but particularly challenging trails, once they've adapted to their condition. They may, of course, be more spooky.
Blind horses should not be expected to jump or do other tricky obstacles. (Some blind horses can jump, but it's rare). They can be taught to go over most trail obstacles by a specific verbal or tactile cue. Some blind horses, but not all, prefer to follow a sighted companion on the trail.
Many well adapted blind horses do very well at dressage, where they are worked in a level arena and expected to follow the rider's cues exactly. One horse, Valiant, competed successfully at fourth level despite being totally blind. I've also found stories about blind horses running barrels.
Unilaterally blind horses - those blind in one eye - can do anything a fully sighted horse can do, although they may need more help judging when to take off for a jump.