French blackout consists of four layers: the face fabric, white interlining, black sateen, and white lining. If the black sateen is a tight, dense weave, it will block all light from coming through. There still might be dots of light where the rings are sewn, but they're fainter than with regular plain blackout lining. And the four layers make the shade wonderfully substantial.
Interlining is layered first, then tightly woven black sateen. The black provides the opacity to keep the light from coming through. White interlining separates the black sateen from the face fabric to keep it from looking ashen.
The prepared lining is laid out and hand slip-stitched to the side hem, then rings and ladder tape are sewn at each rib pocket. We press the pockets downward, and catch the pocket with the ring stitch through all layers.
French blackout, whether used for shades or drapery, is more labor intensive, but the result is luxurious and effective. This shade feels like a piece of furniture!