A blind spot was a location where a security system or sensor could not detect an intruder.
When William T. Riker participated in a battle simulation at Starfleet Academy, he calculated a sensory blind spot on a Tholian vessel and hid within it. Many years later, in 2365, Lieutenant Commander Data noted this as one of Riker's various battle techniques while preparing for Operation Lovely Angel. (TNG: "Peak Performance")
The magnetic storm at the magnetic pole of a planet in the Delta Quadrant was called a blind spot by Harry Kim. At this location, the orbital sensor net was unable to detect the USS Voyager. (VOY: "Resistance")