Proctor's use of the term "black mischief" is more designed to address what is happening in Salem surrounding the Witch Trials.  The idea of what the girls did was mischievous in its own right.  Yet, Proctor's use of the term is meant to bring out how others in the position of power are manipulating that for their own benefit and their own use.  The idea of a "black mischief" is something that Proctor is going to pick up again in the end of the Third Act when he suggests that there is a blackness in Danforth's heart and the "blackness" in his name is recognition of the sins that he has committed with Abigail.  In using the idea of a "black mischief," Proctor has been able to bring out how the mischief of the girls has taken on a more evil and malevolent form where there is little hope of redemption.  It is the very essence of recognizing how the actions of the girls have become fodder for so many that the term "black mischief" reveals itself.  At the same time, it is a statement of how those outside of the girls have taken it upon themselves to make sure that what the girls have done can be appreciated and maximized by those in the position of power.  Here, the girls' mischief becomes morphed into something else that is more ominous and dangerous.