Isn't blockchain a kind of distributed proof? You can now say that this user in reddit is related to this post in hackernews via blockchain. Of course, there's no way to prevent a bot to create multiple accounts in multiple websites but I am to understand that there's a cost to this.

OK... I'm not understanding what that would solve, though. That addresses people posing as other people (and you don't need blockchain to do that), but doesn't address identifying AI commenters.

You can always augment your account with your government ID or some sort if you are famous.