A light blocking system is disclosed, which is adapted to be used in a vertical blind which includes a headrail, a control mechanism provided in the headrail, and a covering assembly including a plurality of slats. Each of the slats is connected to the control mechanism with an end thereof, and therefore is hung below the headrail. A gap is left between the covering assembly and the headrail. The control mechanism is adapted to turn the slats in situ relative to the headrail, or to move the slats back and forth in a longitudinal direction of the headrail. The light blocking system includes a cover plate provided corresponding to the gap, wherein the cover plate covers the gap when the covering assembly is in a closed state.