odd event (blue paper) during lead nitrate with sodium iodide makes lead iodide

odd event during lead nitrate with sodium iodide makes lead iodide

I was having my students do the above lab when they ran out of time.  I had them start filtering the precipitate in plastic funnels and covered the funnels with copier paper to keep anything from falling into the funnels to add weight to the solids.  When I came back over the weekend the copier paper was still in place and the filter papers were dry but the copier paper had a blue to dark blue circle where it was covering the funnel.

Any idea what caused the color change on the copier paper?  The lead nitrate was made during the lab by reacting lead granules with concentrated nitric acid. So the solution would be acid, have sodium and nitrate ions, and precipitated lead iodide.