Black Backing is a light-absorbing, cotton/spandex blend that is needed when you are projecting onto an acoustically transparent screen. Without it, light will pass through the screen and reflect off the wall. The reflection will cause a second image to be seen which has a double affect on the viewable image.  The reflected light will also wash out the image.

The Black Backing combined with the Woven Acoustic screen has a sound loss of +-0.2dB, which is less than what the human ear can distinguish.