the tendency to overlook things or people that are closest to one's own immediate environment

他在全世界寻找那个秘密，却没发现线索就在自己办公室里，真是灯下黑。

the physical phenomenon where the area directly underneath a lamp remains in shadow

虽说屋里灯火通明，可就在灯座下面偏偏是灯下黑。