Macie's vulva isn't nearly that recessed, but she gets darker staining like that on occasion, too--and seems itchy. She has a similar issue with the creases in her jowls--they're very deep and catch moisture and dirt and get very dark and itchy. A dilute (1:3) solution of apple cider vinegar and water helps with the jowls, but I've never had the guts to try it around her vulva--strikes me as maybe causing a little too much stinging if the skin is irritated.
But I've been tempted when she seems the most uncomfortable (normally during the spring thaw when it's damper outside).