Mrs. Black was a warlock who, along with her sister, Mrs. Dark, worked for the Magister, for whom they helped supervise several gambling dens for the Pandemonium Club. Collectively, she and Mrs. Dark were referred to as the Dark Sisters, a title they reveled in.
Under orders from the Magister, and with support from Nate Gray, the Dark Sisters escorted Tessa Gray from Southampton wharf to the Dark House. The sisters immediately went to work, blackmailing Tessa into learning about her power to Change, telling her that they had Nate and would harm him if she did not do as they told her.
Some weeks after, they proved successful in training Tessa to perfect her ability. They deemed her somewhat ready for her union with the Magister.
Eventually, Shadowhunters of the London Institute infiltrated their home in their investigation into the death of Emma Bayliss, a mundane the Dark Sisters had a hand in killing. The Shadowhunters found Tessa, who then attempted to escape with them. Mrs. Black and her sister caught up and fought them in an attempt to stop them. During the confrontation, Mrs. Black was beheaded by Will Herondale.
After her death, Mrs. Dark became dedicated to finding a way to bring her back to life and was attempting to perform necromancy on her head when Will and Jem came to her hideout in Highgate. Mrs. Dark was killed before she could resurrect her sister.
After her sister's death, however, Mortmain reanimated Mrs. Black's head and brought her back to life. She still lacked a body, however, and her head remained decayed. For support, her head was kept impaled on a spike—a column of metal with protruding sticks that served as her limbs.
Mortmain promised to give her a new body—specifically an automatons'—for her continued obedience. In line with this, Mrs. Black once again kidnapped Tessa for him, tricking the Shadowhunters into letting her carriage into the gates of the Institute by trailing right behind the Silent Brothers' which was transporting Jessamine Lovelace.
Tessa knew instantly that Mortmain would not keep his word. True enough, Mortmain eventually gave Tessa Mrs. Black's head and metal body parts as a "gesture of good faith."
Mrs. Black was a short and plump woman, with small, pale, deep-set eyes. Her warlock's mark was her elongated taloned hands, which she hid in her gloves that matched her usually bright clothes.