Black mass? What's that, some sort of weird, satanic ritual?
Actually...yes.
Technically speaking, a black mass is a satanic religious ceremony loosely based on the traditional Catholic mass. We don't want to go into too much detail, but suffice it to say it sounds straight off a death metal lyric sheet.
So what does that have to do with a bunch of gangsters? Is Whitey Bulger lowkey funding a satanic church on the side?
Not quite. Instead, the "black mass" referred to by the book's title is the dinner party where John Morris utters these fateful words to Flemmi and Bulger:
"You can do anything you want as long as you don't clip anyone." (2.13.43)
It's the night he gives them permission to commit any crime short of murder.
Besides sounding cool, the term "black mass" reflects how Connolly and Morris have subverted the proper relationship between the FBI and informants—the same way that a Black Mass subverts a traditional Mass. It's a wonderfully dark way to show us how twisted and evil this whole arrangement has become.