In The Princess Bride, Miracle Max says that "as we all know, 'to blathe' means 'to bluff'." I've never seen any other references to the word "blathe"; is this just part of the joke?
It's part of the joke. He clearly heard him say "true love" (as did we all) but he concocts some nonsense about him saying "to blave" and then makes up his own definition of the word.
There's absolutely no indication that this word means "to bluff" in English, old English or any other language known to Google.