I believe that slate blue legs are lighter or darker because of the density of the outer layer of skin, the epidermis. The dark color in most varieties is only found in the inner layer, the dermis. The exception is blacks and blues and brown reds, where the feather color seems to result in more or less melanin pigment in the epidermis. You can check it when posting a bird by cutting through the outer layer of skin at the juncture of thigh and shank and peeling back the outer layer of skin. It is easily done.