The musical is fun,
but I completely forgot that I had seen the movie too. It wasn't until the final scene, where Sara turns and attacks Alfred, then I suddenly recalled a scene almost just like it. Just a bit different.
It was a Dutch version I saw, a bit of a shame as I do prefer original songs and plays but it was quite enjoyable, well paced and well played.
I don't see the ending of the musical as that unhappy an ending. Granted, not a classic happy ending where the hero saves the day, far from it, but not that gloomy.
I definitely have to look for this movie, it's been too long.
http://www.nps.gov/history/delta/blues/