Blind spot is the area behind and to the side of a vehicle that is hard to see in either the side or rear view mirrors.
Blind spot is the area of the petina where the optic nerve leaves the eye. This area is insensitive to light and the eye is therefore 'blind' at that point. Also used now to refer to an area of someone's (otherwise rational) beliefs which is impervious to change through objective information or rational argument.