I have no knowledge about chicken blindness, but with a rapid internet search I don't see any obvious links to major disease, but I do see a link to nutritional deficiency. This can also cause lethargy and lameness in birds, though I'm not suggesting her lameness wasn't caused by the dog.
Knowing how she was injured by the dog would help to better her odds. I had a chicken get chomped by a dog once; it grabbed her around the back and bit into her ribs, under the wings. Her back was all messed up and everything was bruised and green. I didn't know beans about herbal healing or alternative medicine at the time, but I used tea tree oil on her wounds every day, kept her in a carrier, and she healed and lived a few more years before dying of internal laying. I'm not saying to use tea tree oil. I'm just saying that's what I did in that case, but I'm also saying I didn't actually know what I was doing, it just happened to work in that case
So can you identify her injuries? Do you have photos? Is she bruised or punctured? If there is a nutritional issue going on that would complicate her healing. Maybe a good starting point would be to get some vitamins down her. One of the articles I glanced at said blindness can be caused by Taurine deficiency. I don't know how true that is but it may be worth reading up on and verifying.
As far as adding a bird, every flock is unique in dynamic. I've known people
who keep hens that will KILL any new bird they try to introduce, no matter how much controlled introduction they tried. (imo
, that hen should
be culled, not worth the headache, but not my flock!). My first chicken flock grew a few times in bursts of 2-4. At one point I had 6 hens who had integrated flawlessly with one another and had lived together for a year, and out of nowhere they were all bullying the smallest hen so badly she was reduced to hiding in a tree. Nothing I could do would stop them. I tried adding 3 more hens and it 'kinda helped' but it was still out of balance. I finally added my first rooster, for 10 birds total, and the hen-hen aggression stopped in its tracks. Fastforward many years and thousands of birds later, I've never had trouble since, but I've also never been rooster-less since, because I breed the birds. I'm not saying a rooster is the ideal solution, nor the only solution, nor that it would work for everyone, but that helped my situation.
Then take my neighbors; they have their first flock of 4. They tried to add a grown hen and their girls were out for blood on this new hen. So they gave the hen to me and kept their 4. Then they raise a bunch of male meat birds in an isolated pen this spring, and one ends up being a hen, so they experimentally throw her in with the 4 ladies. Right as rain. The girls don't care, don't bully her at all. So then they added a rooster to the mix and hey, everyone's chill with him, too! The 4 girls liked these 2 new comers I guess!
Birds are silly creatures. The best you can do is try it out, be cautious, take whatever precautions you feel you need to, and watch their interactions. Pecking order scraps are normal, but my rule of thumb is that there is no need to draw blood. If someone's out for blood it's a problem. If they're just kicking and wrestling, it's okay. And integration can go on for several days, too! Keeping a close eye on the flock dynamics is the best way to keep everyone safe. But with any luck, they'll take the new girl with ease!
Also; while, yes, keeping them together but separated with a fence
is a good way for them to get used to one another- when you actually go to put her physically in with the other birds though, do it at night. Set her up on the roost and let her wake up with the flock. This is usually way better for the dynamics than tossing a new bird in during the day