Who are Mr and Mrs Black? Why does Montag go to there house?

Mr. Black was a fireman. Montag planted books at Mr. Black's home, setting him up in the same way that he, himself, was set up.

Then he stood in the cold night air, waiting and at a distance he heard the fire sirens start up and run, and the Salamanders coming, coming to bum Mr. Black's house while he was away at work, to make his wife stand shivering in the morning air while the roof let go and dropped in upon the fire. But now, she was still asleep. Good night, Mrs. Black, he thought. -