I believe he simply means that your other opponent should read the board/map/battlefield more carefully. It is usually implied that the player in question (blue here) is making a bad decision, failing to notice that you (the third person in the game) are the biggest threat, the common enemy if you will, etc etc
He tries to make blue understand that his actions (or lack of them) is handing you the game.
Do you understand?
Ofc I'm just speculating, it's a fog game. But the thrase he uses does not imply secret diplomacy to me, simplky asking the opponent to read the situation more carefully. I think you misunderstand the word "board" perhaps?
