I am VERY VERY sorry to read about Onyx's sudden blindness. But if the article (the link Debbie posted) is correct, it states...

Bayer Animal Health estimates that the incidence of vision related reports over the past decade in the U.S. is 0.0008%, or 1 report per 122,414 feline patients treated

That is a MIGHTY slim chance! I have used baytril when it has been prescribed for my cats with no ill effects and I would do so again.
I would not go so far as to bash the vet for not telling you about this possible side effect. If they warned you of every side effect that happened less than 1% of the time in tests, your typical office visit might take 3 days.

Best wishes for Onyx to adjust to her new life. She will do fine!