wife of Louis VIII of France, mother of Louis IX (St. Louis), and twice regent of France (1226–34, 1248–52), who by wars and marital alliances did much to secure and unify French territories.

Blanche was the daughter of Alfonso VIII of Castile and Eleanor, who was the daughter of Henry II of England. Her grandmother Eleanor of Aquitaine, queen of England, traveled to Spain to take the 11-year-old Blanche to France, where a marriage treaty was concluded with Louis (Louis VIII), the young son of King Philip II Augustus. This politically motivated marriage had been arranged by Blanche's uncle, King John of England, and was celebrated in 1200 at Portsmouth, Hampshire. It represented only a brief truce in the struggle between England and France for control over certain French territories.

Blanche, who became French through marriage, was gradually to become French in spirit as well. Although she did not cease to be concerned for her family, among them her uncle John and his allies, her brother-in-law Ferrand of Portugal, and her cousin Otto of Brunswick (later Holy Roman emperor Otto IV), she rejoiced at the French victory over Otto and the English at Bouvines in 1214, marking the first stage of French unification, a goal for which she was constantly to strive. In the same year, she gave birth to Louis, the future king of France. Upon John of England's death, Blanche boldly tried to seize the English throne: in 1216 Louis of France invaded England on her behalf. The English stood firm against him, and John's nine-year-old son was finally crowned Henry III.

A devout Roman Catholic, Blanche soon became involved in what she sincerely believed to be a holy war against the heretical Cathari, a sect founded on the belief that good and evil had two separate creators, which was flourishing throughout southern France. Her husband, who became Louis VIII in 1223, took part in a crusade against the Cathari but suffered a fatal attack of dysentery upon returning to the north of France in 1226. In accordance with her husband's will, Blanche became both guardian of the 12-year-old Louis (Louis IX) and regent of France. She zealously pressed to have Louis crowned immediately, and the coronation took place at Reims three weeks after Louis VIII's death.

Her most pressing problem was to deal with a rebellion of the great barons, organized by Philip Hurepel, the illegitimate son of King Philip II Augustus, and supported by King Henry III of England. In the face of such adversity, Blanche showed herself by turns a delicate diplomat, a clever negotiator, and a strong leader. Dressed in white, on a white palfrey draped in the same colour, she rode into battle at the head of her troops. After an attempted abduction of the young king, Blanche did not hesitate to replace rebel noble associates with commoners if she thought it necessary. She also created local militias. Blanche was gradually able to subdue the revolt, establish a new truce with England, and, in 1229, pacify the south of France by signing the Treaty of Paris with Raymond VII, count of Toulouse. France then entered an era of domestic stability, which saw the construction of many cathedrals throughout the country.

On only one occasion did Blanche fail to exhibit diplomatic conduct. In 1229 a dispute between an innkeeper and some students took place in the Latin Quarter in Paris. The police were summoned, and the students were beaten and thrown into the Seine; such intervention in the Latin Quarter, however, was contrary to the prerogatives granted to the university, and the faculty and students threatened to strike if the university's privileges were not respected. Badly advised, Blanche held firm, but the university closed its doors, and the faculty and students left Paris for the provinces and abroad. It was to take four years and the intervention of the pope before the university would return to Paris with new prerogatives, this time granted by Blanche herself.

Although Louis IX came of age on April 25, 1236, Blanche remained at his side as his most loyal and steadfast supporter. She lacked tact, however, with regard to her son's private life. Although Blanche herself had selected Margaret Of Provence to be Louis's wife, she treated Margaret with considerable severity. In 1244, after Louis recovered from a serious illness, he and his wife, much against Blanche's wishes, made a vow to go on a crusade against the Muslims. They embarked in 1248, and once again the kingdom was entrusted to Blanche. Informed of Louis's defeat at Al-Manṣūrah, Egypt, and his subsequent imprisonment, Blanche herself went to seek his ransom and that of the French army. She petitioned her parents, her allies, and the pope for funds and supplies, but interest in the crusade had dwindled.

Although weakened by a heart ailment, Blanche did not neglect her obligations as a regent. Continuing to preside over council meetings, she signed laws and watched over the poor of Paris. When some of the poor were mistreated by the cathedral chapter, she herself rode, as formerly, to open the gates to their prison. On her way to the Abbey of the Lys, one of her favourite retreats, Blanche suffered an attack of the heart ailment that was to take her life. She was returned to the palace of the Louvre, dressed in a nun's habit, and laid on a bed of hay. There, after begging forgiveness of all and having received the last sacraments, she died. She was buried at Maubuisson Abbey and her heart taken to the Abbey of the Lys. Louis IX was in Jaffa when he learned of his mother's death. The news distressed him greatly, for he was aware that he had lost not only an incomparable parent but also the strongest supporter of his kingship.