What are Blackout Curtains from World War 2?

Homes, businesses and factories all had some curtain or even painted Windows that wouldn't allow light to be seen from outside at night as a protection from night bombing raids. So, as soon as the lights were turned on the blackout curtains were drawn to make sure no light was visible to an outside source. If you look around at some of the really old factoies you'll notice some of the window panes are still painted.