If it were the shutter hanging up, that edge at the black hole horizon would be sharper, so I'm going to go with the mirror theory as well. It's also in the correct position to be the mirror, as the image on the film is upside down. Does this camera have a mirror lockup? If not, when doing your look-through-from-the-back-with-no-lens test you could just hold the mirror up with your finger. With the mirror out of the way, do you see the whole frame like you expect?

Duncan