A dark-out apparatus is provided for use with a ventilation fan disposed on the wall of an agricultural building for passing ventilation air from one side to the other. The dark-out apparatus includes light-trapping media and first and second door members for holding the light-trapping media and being movable between open and closed positions and being openable away from the wall. When in the closed positions, the first and second door members cover a ventilation port of the fan, and when in the open positions, the first and second door members expose the ventilation port. When in the closed positions, distal sides of the first and second door members define an air gap. The air gap allows ventilation air to pass through, but does not allow ambient exterior light to pass through.