Black mischief is what we today would term "Black Magic." Any act of witchcraft, voodoo, paganism, or ritual sacrifice could fall into this category if one believes as the Puritans did. In the case of John Proctor and the other characters in this play, the fact that the girls were dancing in the forest and the fact that Abigail drank the blood charm to curse Goody Proctor would be considered black mischief. Mercy Lewis, we discover, is the one who had removed her clothing during the near-bacchanalian dancing that occurred in the forest. The girls, suppressed by Puritan society, were blowing off steam, and this act was misinterpreted by others as a form of witchcraft. They also begged Tituba to conjure spirits, which the Puritans had a strict taboo against. This act was also considered black mischief.
The term black mischief in the play "The Crucible" is used to represent the acts of engaging in devil worship, voodoo, and witchcraft. To the Puritans any of these acts was part of what some now call black magic.
In the book the girls had danced naked in the forest which was part of pagan rituals. Anything pagan would be considered black mischief. They drank chicken's blood which was also a ritualistic activity against the Puritan’s beliefs.
One object that came to question as an article of black mischief was the doll that had been at Goody Proctors.
Abigail lies and says that she had terrible pains because Goody Proctor had used it to hurt her by sticking pins in it.