Blanche of Navarre, however, proved to be an incredibly strong and efficient regent, and had devoted herself for the past 15 years to ensuring the legal status of her son Theobald IV as rightful heir.  Blanche was aided in securing her powerbase during the first few shakey years of her regency by the fact that so many lords and knights of the county, who might have posed a challenge to her, had left to fight on the Fourth Crusade from 1202 to 1204.  Queen Mother Adèle of Champagne  also took Blanche under her wing, giving Blanche vital counsel during the early years of her regency until Adèle's death in 1206 . By 1216, despite the fact that Erard rallied most of the local barons of the county against Blanche, she had built up such strong alliances with both King Philip II as well as Pope Innocent III that Erard never gained any official legal support for his claims.  From the start of the conflict, Pope Innocent III began excommunicating rebel barons, negatively affecting their efforts.  Further, while Theobald IV was still an underaged youth of 13 years, he had acquitted himself so valiantly in combat at the decisive Battle of Bouvines that King Philip II threw his full support behind him .
Question: Who did Blanche of Navarre think was the rightful heir?
Answer: her son Theobald IV