In the film industry we use a very cheap and very opaque product to block out windows. We often need to shoot [indoor] night time scenes during the day and can’t have any stray light.
Product is called Duvetyne, it’s a very, very heavy black cloth. We even use it for flags and cutters, which are light-blocking pieces that we put in front of lights as big as 20K (20,000 watts) to deflect and control stray light. This stuff works great.
Has it for $8.25 per yard (60″ wide), so it really is cheap as dirt. You can buy a 50 yard roll for a little over $400, which has got to be enough to do the windows on two or three average houses. At that price I wouldn’t want to be using old rags and what have you. I hope that this helps. – Adam