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

Mrs. Black is the wife of Black, one of the firefighters Montag worked with.
In the story, we first come across Black during a conversation at the fire station. In response to one of Montag's controversial questions, Black and Stoneman (two firemen) open up their rulebooks to reveal the first five rules of firefighting. We later discover that the rulebooks are a means of facilitating the government's fight against independent thought.
Montag's question is telling, as it demonstrates his rising dissatisfaction with his job. He hints that he much prefers to do what firemen traditionally did, namely, put out fires. From the text, we learn that Montag is often accompanied by Black and Captain Beatty as they work on destroying the homes that harbor secret stashes of books.
Later in the story, Black participates in the burning of Montag's home (after Mildred reports Montag). Montag ends up killing Captain Beatty but merely knocks Black and the other fireman out. Later, Montag makes his way to Black's home. When he gets there, he wonders aloud whether Mrs. Black is inside sleeping. Montag then plants some books in the Black family kitchen. It is intimated that Montag is the one who calls the authorities on the Blacks. He wants the Blacks to be caught unawares about the destruction of their home—just as many others had been.
In the story, Black is the quintessential follower who never questions the actions of his government. He burns houses because it is required of him. By extension, he and Mrs. Black are representative of most of Montag's neighbors. They are content to protect the status quo and are fearful of what the consequences of rebellion will be.

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.