Flashbacks scenes in Movies/TV should be depicted as being more blurry than present scenes, since most people can't actually remember a past event that accurately.

Aschtuàlly ! Some movies do this

Sometimes they're desaturated.

Sometimes they're 3:4.

Sometimes they're straight-up black and white.

Changing something about the appearance of of the image is done often. But I don't think blurry would do unless it's a really short flashback. It would either look like a mistake (camera not in focus for instance), not be intense enough to be apparent on smaller or lower resolution TVs, or be distracting and annoying to the viewer (say, some sort of foveated motion blur).

I can't remember the last time I saw a flashback that didn't have a filter on it

hmm.... i guess without spoilers, the boy and the heron have an important scene like this... visually stunning..

They kinda do. Aren't memories typically depicted with a kind of hazy camera filter over them?

Some don't do it to confuse the viewer and create a surprise when the viewer knows that it was in the past

Bojack Horseman did some interesting stuff to represent a character with dementia's failing memory as she tried to recall things. Unimportant or painful to recall people's faces were blank or scribbled over, the background of scenes got vague sometimes, and one memory would visibly collide with another in some confusing mix.

Or what if a character sustains a head injury and scenes after it are in black and white and scenes before it are in color. I was going to make a joke here but this is Memento.

They're primarily a tool to remind audiences what happened/is relevant, but it'd definitely be interesting for a show to unceremoniously alter flashbacks in similar ways to how brains alter real memories.

Also they're generally in the 3rd person which makes no sense

It's as if Kurosawa never made Rashomon.