Blue checkmark is not a sign of authenticity since Elon Musk bought twitter, now it means that the account owner bought twitter subscription. But even if the checkmark still was about authenticity, it would change nothing - no one knows Satoshi's identity, so no one can verify it.

To me even just a signed message is not a full proof, because there can be many ways how someone would get their hands on those keys. Someone who claims to be Satoshi would need to present a lot more evidence than that - a full story of how they created Bitcoin with some small details, etc.