As in заря? That'd make for Twilight Dawnlet.
I found one slavonic dictionary and apparently "зеркъ" is an adjective which meant blue-eyed, but I wasn't able to find any other sources for this word, I've never heard it before, and it makes little sense both grammatically and by meaning.
You might be on the right track, given that afaik Ukrainian generally tends to be descended from a similar ancestor to Russian, and is closely related to old slavonic. It's possible there's a common root that did exist in slavonic but got lost in Russian, and I just haven't found it.