That rats with black coats almost always open their eyes a day or so before rats of other coat colors?
I've raised quite a few hundred litters and I've never noticed this to be true.
My litters have various colors in most of them that include blue, agouti black and Siamese and combinations of those also, and I don't really see black rats doing anything different than the other colors pertaining to when they open their eyes.
Well my experience is quite limited, as i've only raised about 35-40 or so, but i've noticed this in every one of my litters. I just found it weird lol.
It might just be a streak your on?
If you've been noticing it maybe keep track of future litters for awhile and just mark the colors that open their eyes first and see if over time your theory pans out, who knows you might just be right