There have been several movies based on or involve the music and themes of the Blues. Most of them have been very good, and worth watching (none of that fluff we're used to with romantic-comedies etc).
So here's an essential list of some of my favorites:
Needs no introduction.
BLACK SNAKE MOAN
Anything with Samuel L. Jackson is worth watching anyways.
Steve Vai versus the Blues?!?! HELL YEAH. The below is the famous "guitar duel" from the movie.
A good representation of the Chess Records era of the Blues, showing what Muddy Waters, Howlin' Wolf, Little Walter, etc went through.