Snape gestured Narcissa to the sofa. She threw off her cloak, cast it aside and sat down, staring at her white and trembling hands clasped in her lap. Bellatrix lowered her hood more slowly. Dark as her sister was fair, with heavily lidded eyes and a strong jaw, she did not take her gaze from Snape as she moved to stand behind Narcissa.
Harry Potter and the Half-Blood Prince
I don't understand the part "Dark as her sister was fair". What does it really mean?