I had chickens for 18years before my first time with this. Coach723 really helped me, it was the same, puffy lethargic one chick and then after drench, completely better the next day and I followed the instructions treating the whole flock of 5 littles. Then the next year the moment I saw one...
That's very encouraging. If you see her zipping around by tomorrow, you'll be certain that was it.... Just keep up the water dosing for 5 to 7 days for all.
You may be right that she's blind but if that is not the case I would not give her anymore Vit B, start corid with the whole flock, giving her a drench. https://www.backyardchickens.com/threads/corid-oral-drench-instructions.1211991/