to look at, it's plain white sheets coming off the roll (bolt?) (I think 66" wide)
So, being white, it'll reflect light within the room as well, it's not actually black. If you want to control the light in the room itself you may want something else covering it.
But blocking out sunlight (and the heat it would bring) it works great. I've had this up on all my bedroom windows at my apartment for some time first to reduce cooling needs (horribly inefficient apartment, hardly any insulation) but now that I work nights its even better to have a dark room to sleep in (since I sleep during the day, and blinds never block all the light).
To hang this, you'll need to sew a flap over (had my mom do this, since I haven't a sewing machine) so that you can run a rod or such through it to hang it. Alternatively, you can hang it up with staples or push pins or something. Just hanging it there'll be gaps at the sides (hanging away from the wall normally) and bottom (due to sill making it not sit flush to wall). But you can hang it to put it up easily, then pin the sides flat to the walls so that there's no gaps.
It's pretty effective at blocking light and heating from the sun - it's useless to prevent heat escaping (it's blocking the incoming light/radiation from the sun that brings heat, but has no insulation value inherent to prevent heat from crossing the material from conduction. (it isn't helping to keep the place warm in the winter - I'd be better off removing it and hoping for a little solar heating).
Walmart also carries prefab BC ready to hang but its like $25 for one window's worth. Easier, since its premade for hanging, but more expensive.