These words are spoken by John Proctor; ironically, too, he uses a term for witchcraft and places it upon the very people who are accusing others of the
black arts." And, yet, these trials are truly "black mischief" in the sense that they are based upon falsehoods.
This line taken from Act II is part of the dialogue between John and Elizabeth Proctor. Elizabeth tells John that their servant Mary Warren has gone to Salem because she is "an official of the court." Astonished, Proctor asks his wife "What court?" Elizabeth replies that there are fourteen in the jail, and the Deputy Governor promises to hang anyone who does not confess to consorting with the devil when the others have been bewitched by them, having fallen to the floor.
In his attempt to scoff at the lies that have been told, and facts that have been distorted and exaggerated, John Proctor cries out, "Oh, it is a black mischief!" but he is worried about Abigail and the extent to which she will go to create hysteria.