How does the author use images of darkness and light in the section titled, Saint Marie?

In the section titled, Saint Marie, the nun's habit is both black and white, making her goodness or badness ambiguous. Satan is described as "the Dark one." A holy man's black hat carries smallpox. When Marie is locked in the dark closet, the only light comes from the bottom of the door, and then Leopolda lets the light in.