Her full title is usually said to be Dark/Mysterious Lady of the Nine Heavens. She studied at the feet of the Queen Mother of the West, Hsi Wang-mu.
During the battle between Huang Di and Chi You, it is said that Xuan Nu came to Huang Di, either with her sisters or alone, and taught him many kinds of magic, writing, and scholarship, which he then passed on to humanity. Notably, some of this teaching was said to happen in the bedroom, and is sometimes cited as the basis for sexual alchemy in Taoism.
Xuan Nu is one of the goddesses I approached in the search for a name for my Dark Lady, and overall she was probably the Lady I worked with who was the closest match. She's extremely knowledgeable, one of the most widely studied gods I've had the pleasure of working with.
My experience of her was that she could be quite demanding of her students (though I think that's understandable) and I would no recommend approaching her lightly. She will push you further than you think you can go. Sometimes that's a necessary thing.