How well do 'blackout' smart shades actually block light?

True 100% blackout requires both an opaque fabric and side channels or a tight reveal mount; without channels, owners and reviewers consistently note light bleed at the edges even on shades marketed as 100% blackout. The shades in this roundup labeled blackout use multi-layer fabric that blocks the fabric face fully, but if you need cinema-grade darkness, plan for a deep inside-mount or add side tracks.