In short
An area around the car that the driver cannot see using mirrors alone; requires an over-the-shoulder check.
A blind spot is a region around the car that is not visible in any of the mirrors. For a typical car, the main blind spots are at the rear corners on each side.
Examiners watch for over-the-shoulder blind-spot checks before pulling away from a kerb and before changing lanes — particularly when changing right (offside) lanes on dual carriageways.