At 39 I am not sure I would have even brought in another horse; if she'd been fine previously, why mess with her state of mind? That she's blind, as you've found out, further complicates things, and so further upsets her!
You could have tried a goat, sheep, or even a llama; these would have made perfectly acceptable buddies as well, and they wouldn't have tried to dominate her, and thus she wouldn't have been so 'turned upside down' by the process.
Of course, now, you've got another horse, who knows how to pick on her, and make life miserable for her. What fun is it for either horse to live a solitary life, when you got the other one for companionship for the other? It's up to you, but I would return him, or find a good home for him yourself, and look for a different sort of companion.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."