The case with your dam must be that the "silver factor" is hidden (probably not the right word) by the white. And shows why White Rocks can be used with red birds to produce Red Sex-links.
It would be interesting to know if all the color is lost to white as the chick matures.
I don't know much about this sort of cross but maybe others will comment.
I don't particularly know the genotype of white rocks but I'm guessing that they have yellow legs. It looks as if the white rock has barring under recessive white to help restrict the pigment from the legs thus when crossed the recessive white would not show (as it has one recessive white gene) & the barring on black would show. Following that train of thinking that chick would be a male.