It's mother could be an olive egger? Theres a chance that even because it's parent is an OE that they don't lay an colored egg, and that could explain the color!
So that bird probably isn't a polish mix based on the shape, but I'm not thinking Wyandotte either. Single comb is recessive. Looks similar almost to Brahmas 'dark' variety...
So you want to make another bird like her? Similar in color only or you wanna try to make Her2.0?