From behind, they look like what might be a pretty young maiden with very long black hair, wearing either a beautiful kimono with many layers or the usual "ghost" attire, a simple white kimono. She is seen holding a mirror or a fan - in which case she is sitting or standing near a mirror. It is when the onlooker comes closer and sees her reflexion, or when she turns around, that her real appearance strikes them.
The Ao-nyobo is indeed depicted as having the scariest face to look upon. Sometimes wrinkled, but always with shaved eyebrows (and fake, painted ones, instead), sometimes with blackened teeths and sometimes with long beast-like fangs, she always manages to be especially terrifying.
Vain and petty, the Ao-nyobo believes firmly that she deserves a handsome, rich, strong young man as husband. But since her appearance scares Humans, who run away or attack them, they quickly get angry and devour them as a mean for revenge. They are also known to be jealous towards pretty young women.
Ao-nyobo as a general term referred to a young court woman who had not yet achieved a very high rank. Nyôbô usually served in the imperial palace for a while until they found a decent match and left to get married. So how did a prime example of youth, beauty, and refinement get turned into a yôkai? There seem to be a lot of different versions of the Ao-nyobo story, but most seem to deal with love lost, waiting, and vanity (they are often picture holding a mirror or sitting in front of one), all of which relate to the position of these women.
Court women had their own residences, but they were supposed to be hidden behind screens and unseen by outsiders, so they simply had to sit and wait for their husbands or lovers to come to them. Many stories tell of women waiting for a lover who never comes, having found a wife somewhere else, or died in battle.
The Ao-nyobo was one of those women, aging and growing less attractive but never advancing in rank or finding someone to "take her away from all that", until they died. Being vain, she continues, even in death, to put on makeup, gazin into her mirror, sitting alone, waiting until some unlucky man comes along... then devours them.