a/somebody's blind spot
1. An area that is not able to be seen, either due to its location outside of the field of vision, or due to some physical obstruction or a defect in one's vision. Often used to describe the areas around a car that cannot be seen with the rear-view or side mirrors. This car has a big blind spot on the right side, so remember to always turn your head to look before merging. Leslie always experiences blind spots when she starts to get a migraine. That security camera has a blind spot—and that's where we'll sneak in.
2. By extension, an aspect of one's life or a certain situation that one is ignorant of or that one does not understand fully. Jen has a real blind spot when it comes to her relationships with men trying to take advantage of her wealth.
Subject about which one is ignorant or biased. For example, The boss has a blind spot about Henry; he wouldn't fire him for anything, or Dad has a blind spot about opera; he can't see anything good about it. This term uses blind in the sense of "covered or hidden from sight." It has two literal meanings: an insensitive part of the retina and an area outside one's field of vision. The phrase has largely replaced blind side, which survives mainly in the verb to blindside, meaning "to hit someone on an unguarded side" and "to deal an unexpected blow." [Mid-1800s]
a blind spot
COMMON A blind spot is an area of knowledge or understanding that you do not have or pay no attention to. The government have a blind spot when it comes to the environment. Patrick's never been great with money — it's a bit of a blind spot with him.
a blind spot1 an area into which you cannot see. 2 an aspect of something that someone knows or cares little about.
These general senses appear to have developed from a mid 19th-century cricketing term for the spot of ground in front of a batsman where a ball pitched by the bowler leaves the batsman undecided whether to play forward to it or back.