I would go with the definition of a blunder as a move that you instantly recognize as wrong once you see the response. So the definition is relative to your own

I would go with the definition of a blunder as a move that you instantly recognize as wrong once you see the response. So the definition is relative to your own skill level. It's at least partly a property of the game itself. If the game is tossing a coin, first to 100 heads wins, then the win probability will not take major swings. If you have a game where the person in a worse position has many opportunities to recover, win probability does not swing so much.