Nothing was spilled on the film. That shot was made from multiple elements. You had the horizon which was a seperate element and the they comped it into the shot of Luke. When they did that it resulted into the blob.
But that blob's not present on every frame of those scenes, is it? Are those the first frames of those two shots? If so, maybe it's something to do with the way they film was originally spliced (there were all kinds of jumps and scrapes and rubbish on first or last frames that I cleaned out of my LD transfer, which weren't there when I got the 2004 DVDs to use as source).
DE