I moved a few weeks ago and really been missing the lighting at my old place. When I removed my old curtain it was filthy and moldy.. yuck! I decided not to bring it in to my new home. So.. that left me looking for something new. I ended up at the "Indoor Gardening" store, and bought some Panda Film. If you don't know, this is the stuff that indoor gardeners to stop light from getting in, and it clocks 100% of the light. It's white on one side, and black on the other, and comes in rolls 10 feet wide.
I asked for 5 feet, but he gave me 10 and only charged me for 5. This is great! My window is only about 3-4 feet tall, so I was able to double the plastic over, giving a clean white appearance on both sides. I put it up with a tiny bit of white duct tape, and upon a quick test is is blocking ALL the light!
I couldn't be happier! Just the other night I read about some $300 black out windows.. yuck!