While blackout shades are engineered for total light blockage, achieving a 100% blackout effect depends on installation. For inside-mounted blackout shades, you can expect them to block over 99% of light. Some light may seep in through small gaps on the sides, top, or bottom of the shade. For a truly near-total blackout, consider adding side tracks or a cassette to your window shades, which minimizes light leakage around the edges.