Yup, Black and White film! Very strange that color comes out of it. I'm speculating, but I was thinking along the lines of silver crystals being precipitated out by such a long exposure, and if there are enough of the correct size, they could absorb longer wavelengths, giving a blue tone were very exposed (and yellow when inverted). But that is merely speculation on my part.
Construction paper is a neat Idea. I've thought of trying that when I noticed that sheets left in the closet for years get brown when exposed. Newspaper might be a bit faster though. It yellows in a matter of days when exposed to bright light.
Originally Posted by holmburgers