Also, you can get blackout lining material used for drapes. That is what I have done. I put small hooks in the molding above the windows and eyehole grommets in the lining material and just hang it over the windows to darken the room.
You did not say what kind of TV you have. If it is a plasma or LCD, you do not need a completely dark room. If it is a front-projector (with a separate screen) the room will need to be almost fully dark.
You need blackout blinds or curtains on all the windows that shed light into the room (includes adjoining rooms windows). Heavy drapes work too. The blackout stuff normally has a white vinyl backing on it.
You don't need it pitch black. A totally dark room can be fatiguing on the eyes. You just don't want light shining on the screen because that can alter the colors and cause glare. Some ambient light is a good thing too if you have to get up to go to the bathroom or grab your popcorn or drink.