Why is my "blackout" shade still letting light in?

Because "blackout" describes the fabric, not the installed result. A blackout fabric passes no light through the material itself, but mounted inside the window frame the shade still leaks daylight around its sides and bottom through the deduction gap — the room ends up dark-ish, not dark. This is the category’s most common disappointment, and the fix is structural, not a different fabric: an outside mount with generous overlap (2–3 inches per side), side channels that seal the vertical edges, or a layered day/night setup. If a genuinely dark bedroom or media room is the goal, plan for the mount that seals the edges from the start — the fabric is only half the answer, and the gap is what wakes you at dawn.