I bought black foam core board at an office supply store. It is about $7 a sheet, and each sheet is about 30"x40", maybe? Can't remember the exact dimensions. But, to cover my 2 bedroom windows I bought 4 sheets, and cut them to fit 1/2 of each window.
Then I applied small tabs of velcro to the foam core, and matching bits of velcro to the window frames. So, now they are easy on and off of each 1/2 of the window for light, etc... The foam core completely blocks the light. There are slivers of light that come past the sides of the boards, but if I wanted absolute darkness, then I could have fit them to the window frames in a different way.
I'm thinking about gluing some nice wallpaper or wrapping paper to them to match my room decor.