If you're playing through steam, you don't have a standard Blockland Key.
Not only does the fact that the game authenticates through steam prove this, but all steam users BLIDs are above a fundamental limit on game keys placed by the game itself, which is 98303. Any key that has a BLID associated with it larger than 98,303 it will automatically reject it, and all steam copies of the game have a BLID well above 100k.
So it's literally, physically impossible for you to have a valid blockland key from a steam version of the game.
Even if the game did create a key.dat file (Which I'm very doubtful of), it does not contain a valid key.