Okay, so she's definitely white. There are three varieties of white: recessive, dominant, and silver. Silver is sexlinked)
In reverse order, I don't believe Silver shows in a solid pattern unless it's leakage. She's not dominant white because if she were, at least half of your babies would be white. Thus, she's recessive white. You need two copies of recessive white to have a white chicken. So she's (c/c.)
Since you have a chick that dark, it's probably black instead of really dark blue. You need two copies of black, one from each parent, to make black. So she carries the black gene. Since you have blue chicks as well, she also carries the co-dominant gene splash (or "true blue.") That makes her Bl/bl.
So she carries the blue genes, but the recessive white is hiding them. Her chicks also carry recessive white, but since it's recessive, they don't show it. Does that make sense?