Johnny Depp and Scott Cooper (director), tell the story of Whitey Bulger - an Irish mafia boss - in the most boring way possible!
The filmmakers intended to make a classic like Goodfellas, but failed miserably. Black Mass tells the story of cops and mafia, and the unconventional alliances between them. Johnny Depp tries to method act his way through the film, and as an actor, he does a very respectable job. Besides Depp, Benedict Cumberbatch was also a nice addition - hearing him in a Boston accent was most fun I had watching this film.
The fault of the movie lies with the writers and director. The story lacks thrills, and the characters are unsympathetic. I found myself not giving a damn for neither the good guys nor the baddies. The pacing of the movie is boring, and the colour palette that the cinematographer uses is dark and gloomy (in other words depressing).
So if you are interested in Whitely Bulger's story, do yourself a favour and just read his Wikipedia page... When I was at the theatre watching Black Mass, I thought the time had stopped and I was trapped there for eternity. Score of 3/10 might be somewhat generous...