There is an anime, "Ef: A Fairy Tale of the Two", which puts in evidence a think that I would not expect. Am I right on this one?

How are either of them to blame for her losing her eye in an accident when neither of them were present?

Hiro accepted to go to the beach, so it lead to Chihiro pursuing him and Kei, and having the incident in the process. He injured her indirectly.

I think the implication is that Kei is seen, at least in part, as a malevolent seductress, and because that makes her some sort of villain, she is seen as to be at fault.
Of course, that is just some garden-of-eden style “eve is at fault” bullshit, coupled with the deep seated cultural need to have some sort of culprit to blame for all of your problems.
The reality is of course, that it was just an unfortunate accident.

The anime is quite fanciful and dream like. Almost like some god is involved, for better or worse, or for chance. Manufactured or pre-planned events. I took it as being quite tongue-in-cheek. Accidents happen. Life happens.

It’s subjective, people tend to blame the ones they empathize with the least. If Chihiro was serious enough about Hiro and had discussed this with Kei she might blame Kei for running off with him but if Hiro was leading on both girls it’s reasonable for Chihiro to feel betrayed. The whole thing could also have been a misunderstanding, it probably was in one way or another. Objectively I don’t think anyone is really responsible for anything, there is only blame.

A friend sometimes quotes this:

“I didn’t say it was your fault. I said I was going to blame you!”