In film and novels a blank slate character to me is just bad writing.
They work a little better in video games because of the interactivity but I still largely have the same problem.
In a film I see an actor, thus I cannot self insert because they are not me.
In a novel I want fleshed out and interesting characters, I can't think of a more boring story than one which follows blank characters just so I can self insert.
In video games blank slate characters are often a make-your-own insert, this makes more sense but then they suffer from being terribly dull and often add absolutely nothing to the narrative at all.
Other times the player character may be defined, but mute or often have zero personality so that you can self insert and this, to me, is a waste of a character.
Let's take Link.
He largely has defined iconography, we know it is Link when we see him, even if it isn't a design we've ever seen before, there's enough definitive iconography between his incarnations to help us recognise him each time.
These iconographies give him an identity different from our own and one we didn't create ourselves.
The fact we can change his name in most games doesn't help half of the world's gender either...
Plus Link is canonically Link, the series was naming him canonically from as early as the second game and the series has used his name in other titles since.
Thus, he may be a blank slate, mute and even androgynous sometimes with no real defined personality so that nothing stops us from identifying as him when we play the games...
But I, in reality, am nothing like Link.
I know games are escapism and that I should let the obvious slide but the disconnect is too great for me, Link is too defined, I see Link like I see an actor in a film. I see an identity.
Thus, I have always found Link to be an example of bad writing (I know, in a series with bad writing), but if he were a real 'link' we should have gender and race options by now.
But race and gender options would then not be definitive.
As much as Nintendo don't want to admit it, Link is not a link to the player.
Link is just one example of why I think blank slates don't work.
I don't think they work in any medium.
If a medium is story-based/narrative driven and you have blank slates then that is bad writing, or at the very least, lazy as hell.
It takes little effort at all to craft a generic character in anything. But for me, I indulge in stories because I want to go on that journey with the characters and not as the characters.
And thus I, personally, I don't believe a blank slate character can be utilised well and don't think that effort should be spent trying.
Just craft decent characters.