Use black duvetyne. (AKA: "commando cloth.")

Duvetyne can be bought in swatches 54", 60" or 78" wide and any length up to a full bolt, 100 yds. long.
You can get it at any theatrical supply house such as BMI Supply:

Buy the 16 oz. fire retardant kind. It's a bit more expensive but it's heavier and more opaque so you won't have to use it in double layers.

I blacked out the window in my basement darkroom by taking a 1/2" thick piece of blue Insul-Board (construction grade polystyrene insulating board) and wrapping it with dyvetyne.

Cut the board a little bit smaller than the window frame. Wrap the fabric around the board but leave some extra fabric hanging loose all around the edges. Push the board into the window then use a putty knife or a stick to stuff the excess cloth into the gap between the window frame and the board. Leave enough fabric hanging off the board so that you can get a good packing all the way around.

My darkroom window is completely blacked out. When the lights are dim, you would never even know that there is a window there.

The neat thing about this is that you can take it out of the window when you don't need it and stash it in the closet or behind the sofa. It's very light, easy to carry and it only takes about 3 minutes to install or remove.

If you don't want to use duvetyne you can use any kind of heavy cloth. Even an old wool blanket. The difference is that duve is made for blocking light. If you get good quality, heavyweight stuff you won't have to use two layers. If you use a blanket, be sure to check to see that the cloth is opaque before you commit.

I use duvetyne everywhere. I use it to black out windows. I made a darkcloth out of it. I keep a 3-foot piece of it in my camera case for when I need to unload a camera in bright light. (Just drape it over your lap while you work.) I stapled a strip of it over the hinge side of my darkroom door to keep the light from leaking in around the frame. It also makes great backdrops for photographing subjects where you want a dead black background.