My Paint mare was born blind - that was how I came to own her. Her breeder was going to have her put to sleep, but a mutual acquaintance asked if I would take her, as I had two other blind horses in my care at that time (both of whom had uveitis and went blind later in life, but adapted well and were still going under saddle). I took her, and it was a fantastic decision!
Depending on what caused the blindness, there ARE treatments. In my mare's case, she was born with severe cataracts due to a uterine infection before she was born. There is a procedure to help with that, and there are surgeries for some corneal conditions, too, but those were all outside my price range.
However, my Rose has been nothing but awesome. She's 8 now, and just a great little horse. She's shown (and won!) extensively at the local level - halter, showmanship, pleasure, equitation, horsemanship, barrels, and poles. She's kid and beginner safe, has been a lesson horse, and her last lease-rider was in training for endurance with her. She's been camping numerous times, done pony-parties - the list goes on. She's one of the least spooky horses I've ever had (and the other two blind horses I've had were ALSO non-spooky both before and after losing sight, even at an older age).
I've also known MULTIPLE horses who lost a single eye in an accident, to uveitis, or to cataracts and I only know one who ended up spooky and unpredictable - a tb mare who was spooky and unpredictable to start with, and it just intensified to where her owner was no longer comfortable riding her.
Here are a few photos of my well adapted blind mare: