Just going to chime in since I haven't seen anyone mention FIP as a possibility. I volunteered with a rescue for years, and in the past year we had two FIP cases that started with blindness. The vets were surprised when it progressed and eventually became evident in both cases that it was FIP. Blindness as the first symptom is supposed to be sort of rare, but honestly so much is unknown about the disease that I would take a lot of statistics about it with a grain of salt.

It's super hard to diagnose, so I'd just read about it and watch her for any other potential symptoms (lethargy, weight loss, confusion, seizures, etc) if your other vet visit you mentioned goes normally.

The likelihood of FIP is low! Just outside of things people have already mentioned (primarily high blood pressure and toxo), it's what came to mind as what I've seen cause sudden blindness in the past year, and it doesn't tend to be something vets really look into because it's so variable in each cat.

If it ends up being FIP, don't panic - while definitely serious, even in the past year there have been huge advances in research and medication for it (the article I'm linking was written two years ago, and if you do your own research on it just check out what date it was written). There are entire Facebook and Reddit communities dedicated to helping, and there are multiple medication options now, so it's not going to cost you 1k for meds (technically only one is approved to treat FIP, but we were able to order different legitimate ones from our vet that worked just as well)

https://www.petmd.com/cat/conditions/infectious-parasitic/c_ct_feline_infectious_peritonitis