blind spot
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]