I picked it because it sounded cheery and because I knew nothing about it, and I was on a kick where I thought it was best to approach film without prior information, so that I could have a first experience of the movie unadulterated by prior expectation.

You're not alone. I did the same thing with a stage show of "Cabaret." All I knew was the song, which is fairly cheery. I felt rather shell-shocked afterwards. Fortunately my date was in the same boat.