Blackout refers to a foam-backed, opaque fabric used to black out light. Blackout fabrics are most use as curtain linings or drapery fabrics, blocking much of the light that would otherwise enter through a window when the curtains are closed.
Blackout save energy ,because blocking heat and cold tranfer the window.
Worked well as a low budget projector screen. Size was actually 54 inch. Was able to stretch a few inches out of it when I made the screen. Was about perfect for a 100 inch screen.