I think the last movie I went in not knowing much was earlier this year when The Descendants game out. I had seen some of the trailer and thought it was a comedy, and I knew George Clooney was in it. Boy I was wrong on the Comedy bit because it ended up being one of the more depressing films I've ever seen.
Well that's not blind because you saw the trailer. It may be bad marketing, but that's not quite the same thing.
I used to do this all the times as an undergrad. I had a 12-month cinema pass, and didn't own a TV or read entertainment magazines. My friend and I would just rock up to the cinema and pick a title that we'd never heard of. We got pretty good at not even looking around at posters inside the cinema (which might have given us genre clues as well as info on who was in it).
One of the most fun parts was trying to gauge what kind of film was about to start based on the audience demographic and the trailers.
I saw some good movies that way. And a lot of utter rubbish. I'd love to take the hobby back up if I can ever afford a pass again.
I like the idea of doing it as a hobby instead of just a fluke. Unfortunately, I don't have the time or money for this, but it sounds like it could be an interesting experience once you've done a couple to know the routine.