A perimeter light blockout system for minimizing light leakage between light gaps such as, for example, between the sides of a covering (e.g., shade) of an architectural-structure covering and the interior side surfaces of a window frame, or between the covering and the outer surface of an interior wall is disclosed. The perimeter light blockout system including a light blocking device and a mounting element for coupling the light blocking device to the interior side surface of the window frame, or the outer surface of the interior wall. The light blocking device may include rear and front channel members. The mounting element may be a mounting extrusion. Alternatively, the mounting element may be a spring clip. In use, the light blocking device may be adapted and configured so as not to contact the covering as the covering moves between extended and retracted positions. The mounting element may include a degree of adjustment so that the light blocking device may be aligned with an out-of-skew window frame.