I think it's the paper you're using, too. I use 300 lb. art paper (with hand-coated emulsions), and it definitely transmits light. Hold your blocker sheet in front of a lamp, to test. My guess is you'll see light coming through. Use heavy cardboard, or (as Thomas suggested) the paper box.
If you'd like to test it on photo paper, cover half the sheet with the photo paper box, and the other half with your blocker sheet. Expose for about 15-20 seconds and develop. If both sides aren't pure white, you have your culprit...