There's definitely a decent story here, and pretty strong character development. It wasn't at all boring, in my opinion. The rising tension is done effectively, and we believe every decision each character makes. Also, the ending was appropriate and not overly contrived.
It did at times, however, feel like the actors were kind of *playing* at being country bumpkins with a tough start in life. That is, the acting didn't feel entirely authentic. The actress who plays Paula, for example, went way over the top with the melodrama. I was rolling my eyes at her a little by the end. I felt similar about the actor who played Noel. Just a little too much screaming and kicking when a bit more subtlety would have been much more effective.
It made me think of the movie Affliction, an indie drama from (I believe, without checking) the '90s, starring Nick Nolte and Sissy Spacek. It's the same kind of themes but much better acting. Anyway, Black Fly is a decent watch even if it's a bit forgettable.
