If there where no opponents there was no reason to be in doubt about your own pure judgment.
You see what you see in a position and that are the only things to be seen.
How often do we hear our opponents who did just loose from us say: "I stood better during the whole game"? With a grin on your face you think "what an idiot, he was worse after move two".
How often do we hear ourself say after loosing a game :"I stood better during the whole game" and when you look at your opponent you see an idiot grin on his face?
Only in an indirect way you can get a notice of something that is there but is not seen by you.
Once you realize that you have a blind spot, with things in it that are seen by other people but not by yourself, the question arises: how big is this blind spot?
Do you miss only 1 % of the things to be seen or do you miss 99%?
I'll try to adress this question.
Against me, Fritz has a score of 100%. I have never won a single game when Fritz played at his highest level.
So for simplicity I state that from my point of view Fritz has no blind spot.
Let's say that Fritz has a rating of 2500, about a (beginning) grandmasters level.
When I let Fritz analyze my games he proposes in 50% of the moves a better one. Which means that in 50% of the cases Fritz "sees" something in the position that I didn't see.
You can only guess what Fritz doesn't see in comparison to "the absolute ultimate moves" of a game.
If we have a blind spot in chess then we have probably a blind spot in life too.
You can get a notice of this when you are aware of people with an idiotic grin on ther face looking at you...