Do blackout curtains really block 100% of light?

Almost never on their own. Even panels marketed as "100% blackout" — including the picks here — typically block 90–99% of light through the fabric itself, but reviewers consistently note light leak around the sides, top, and the gap between panels. The Spruce's testing of similar curtains found about 90–95% light blocking with leak at the panel break. For true total darkness, reviewers across r/hometheater and r/homeownerstips recommend pairing curtains with a wraparound rod, a valance, or side-mounted brackets that hug the wall.