The New Dictionary of Cultural Literacy, Third Edition. 2002.
blind spot
A small region in the visual field (the area scanned by the eye) that cannot be seen. The blind spot corresponds to an area in the eye where the optic nerve enters the retina.
In a general sense, the term is used to refer to an inability to see things that might be obvious to another observer: He has a blind spot as far as his daughters behavior is concerned.