When the knife cut Dark on accident over her right eye, the blood seeped in, causing her vision in her right eye to become a light tint of red. It irritated her a lot for awhile but she's gotten used to it. Though her right eye looks totally normal when you look at it.

Imagine just seeing normal through your left eye and seeing a light tint of red in your right eye. Yea, that would irritate me too.