A blackout curtain can be used as a projector screen in a pinch, but it's not ideal. The fabric will absorb some of the light from the projector, which will reduce image quality. Blackout curtains also don't have a smooth surface, so the projected image will be grainy. If you need a projector screen in a hurry, a blackout curtain will do in a pinch, but for best results, invest in a proper projection screen.