Blancheflor first appears in Chretien de Troyes's Perceval, or The Story of the Grail, written in the 1180s, the romance that created the Grail quest as the medieval world would know it. Perceval, newly knighted by his uncle Gornemant de Gohort, rides out into the world and comes to a castle by the sea called Beaurepaire. He finds the town impoverished, the people starving, and the garrison reduced to shadows. Yet Blancheflor greets him with grace, her eyes glinting blue-grey and smiling, her hair like spun gold, her cheeks the colour of scarlet on silver. She offers him all she has: a freshly shot roebuck, five loaves sent by a prior uncle, and wine so sour it must be reheated to be drinkable. The knights around them whisper that God made each for the other, so fair did they look together.

That night, unable to sleep for fear of the morrow, Blancheflor comes to Perceval's chamber. She kneels by his bed and weeps until her tears wet his face and wake him. When he asks what troubles her, she tells him the story of the siege. Clamadeu of the Isles desires her and has sent his seneschal Engygeron to take her castle by force. Of the three hundred and ten knights who once defended Beaurepaire, one hundred and sixty have been killed, forty-eight captured, and only fifty remain. The castle has been besieged for a full year, the supplies are exhausted, and tomorrow the garrison must surrender. But Blancheflor has a knife of finest steel hidden in her casket, and she swears she will drive it into her own heart before she lets Clamadeu possess her living body. She has told him everything, she says, and now she will let him go back to sleep.

Perceval does not go back to sleep. He draws her into the bed beside him, comforts her through the night, and in the morning rides out to face Engygeron. The seneschal falls before the young knight's spear and begs for mercy; Perceval spares him and sends him to Arthur's court as a prisoner. Clamadeu, learning of his seneschal's defeat, attacks with twenty knights and then with his whole army, but Perceval defeats them all. At last Clamadeu himself faces Perceval and is forced to beg for mercy. He too is sent to Arthur's court, and all his prisoners are released. A ship laden with wheat and provisions arrives by chance, and Beaurepaire is saved.

Blancheflor offers Perceval everything: her hand in marriage, the lordship of Beaurepaire, a kingdom to rule. But Perceval, anxious about his mother whom he had abandoned to become a knight, declines. He promises to return, mounts his horse, and rides away. Chretien never completed the romance, and in the original poem Perceval never does return to Blancheflor. But the continuations written by other hands in the decades that followed gave the lovers their ending. In the fourth continuation, composed by Gerbert de Montreuil around 1230, Perceval returns to Beaurepaire, defeats another attacker named Caridés of Escavalon, and at last marries Blancheflor. In Wolfram von Eschenbach's Parzival, written around 1200-1210, her name is changed to Condwiramurs, derived from the Old French conduire amours, meaning to guide love, and she becomes not merely Perceval's sweetheart but his wife and the queen of the Grail kingdom. In the Middle English Sir Perceval of Galles, she is called Lufamour, Love's Reward, and the story is told with a rougher humour but the same essential shape: the lady of the besieged castle, the young knight who saves her, and the love that follows.