Thought I would also include a picture of Golden Cuckoo Marans leg color (leg color of both silver and golden cuckoo are the same). This is my rooster, but a pullet would be similar, distinctly "pink" like him with a dark wash.
Bottom of foot color is needed, but they look to be Blue Marans rather than blue copper, but the copper color can develop with age. I can definitively tell you that your bird is a Barred Rock. Her legs are not slate. Slate by definition is a very dark grey color when it comes to leg color. Slate...