I have one who is practically fully blind but still has some slight sight in his right eye. I would not recommend jumping him. I would work solely on forming a great bond with this horse. Most horses who go blind and have a good trust for there rider/handler will be unstoppable for pleasure riding when they lose there sight. Some horses who are losing there sight tend to be a bit more spooky then blind horses.
The reason I would not jump him is because of his sight. With out having a full vision the horse could mis jump and get him and yourself hurt. I have however taught my horse to pick up his legs and a certain tap on his legs will guide him to how high he must step, but we are not trotting or cantering we are simply walking over it. It took time and I had to work on this before he went blind and am still working on it.
If you want to compete in jumping I would not buy a blind or part blind horse.