Who is Mrs. Black in the novel Fahrenheit 451 by Ray Bradbury?

She is married to one of the firemen with whom Montag used to work.

In Part Three of the novel, Montag kills Captain Beatty and becomes an enemy of the state. After barely escaping the Hound and crossing a dangerous highway, Montag sneaks up to Black's home. Black is the last name of one of Montag's former coworkers. Montag quietly approaches their house and asks himself if Mrs. Black is home. He wonders if she is asleep and mentions that what he is about to do isn't good. Montag justifies his actions by commenting that Mrs. Black's husband destroyed so many people's lives over the years by setting their homes on fire. Montag then creeps into her home and places several books into her kitchen. Montag then enters a phone booth where he calls the authorities on Black's home. The authorities immediately drive towards Black's home, and Montag is able to escape safely to Faber's home.