Blanche was instead married in 1236 to John I, Duke of Brittany: the main reason he married Blanche was so he could get Navarre, and Theobald did make John heir to the throne.
In 1236, Blanche was married to John I, Duke of Brittany. The main purpose of this marriage was for John to gain control of Navarre, and Theobald ensured that John would inherit the throne.