Blanche of Castile exercised royal authority as queen of France and twice as regent for her son Louis IX. Her marriage to the future Louis VIII in 1200 linked the Capetian and Castilian royal houses. During her husband's short reign from 1223 to 1226 she gained political experience and influence at court. His death left a young heir whose succession required immediate protection against powerful nobles seeking to recover freedom of action.

Blanche arranged Louis's coronation and governed in his name. She used negotiation, alliances and military pressure to defeat coalitions that challenged her authority. Opposition drew on both baronial interests and hostility to government by a foreign-born woman. Her ability to retain loyal supporters prevented those challenges from becoming a successful alternative government and preserved the monarchy's recent territorial gains.

The Treaty of Paris in 1229 brought the count of Toulouse into a settlement that strengthened royal influence in Languedoc. Dynastic marriage arrangements helped prepare the region's later incorporation into the crown's domain. Blanche continued to advise Louis after he began ruling personally, and her influence extended to the royal family's marriages, finances and religious patronage. She supported monasteries and founded Maubuisson, linking pious benefaction with dynastic remembrance.

When Louis departed on crusade in 1248, Blanche again assumed responsibility for government. She maintained order and resources at home while the expedition encountered disaster in Egypt and the king was captured and ransomed. The Shepherds' movement of 1251 posed another challenge as popular crusading enthusiasm turned into disorder. She died in 1252 before Louis returned to France. Her career demonstrates the substantial authority a royal mother could exercise through regency and counsel, but her achievements depended on administrators, clerics and aristocratic alliances as well as personal resolve.