If you plan on using sheets of plastic as a room divider, you're going to want to make a sandwich with black in the middle and white on the surfaces.

Plastic doesn't block light all that well. There's something called PAnda film that's meant for this purpose but if you're just using the type that is used on construction sites plan on multiple layers.

For just covering walls, IMO white paint is easier to deal with. I'd go with an oil based primer that contains a fungicide.

If you want to go super cheap, order some mylar emergency blankets from a camping supply place. You can find them on the internet for like a buck each for a 4x6 piece. But it is VERY thin and does not block light at all, only good for covering walls.