FBI agent John Morris spends the bulk of Black Mass in a personal tailspin of epic proportions, and that tailspin is fueled largely by booze.
For instance, Whitey Bulger and Morris bond over wine, despite Bulger not drinking much himself. Instead, Bulger frequently tries to "ply Morris with fancy wine—here and there a $25 or $30 bottle of French Bordeaux" (2.9.60).
Besides showing Bulger's flashy nature, this rampant gift-giving is a premonition of Morris' acceptance of cash from his informant.
Morris' other big, booze-soaked moment comes after the prosecution of the Angiulo crime family. The crew meets at a hotel and Morris, in an absurd conflict of interest, plays a secret wiretapped tape recording from the investigation for Bulger and Flemmi. But that's not even the worst part.
Take a look:
Besides emptying more than two wine bottles, Morris had stumbled out of the hotel, leaving behind the top-secret government tape recording he had so proudly played for Bulger and Flemmi. (2.8.55)
In both instances, alcohol becomes a symbol of John Morris' decline over the course of Black Mass. Whether used for bribery, or to dull his feelings of shame, Morris' spiraling relationship with booze mirrors his spiraling relationship with Whitey Bulger.