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Blanche was firstly betrothed to Otto III, Count of Burgundy; the marriage contract was signed on 16 January 1236. However, the engagement was broken.
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. However, John renounced the claim after Margaret of Bourbon bore Theobald two sons.
Of their eight children, only their eldest three lived to adulthood.
In 1270 Blanche founded the Abbey de la Joie near Hennebont; she was later buried there. She died in 1283; her husband outlived her by three years. Blanche outlived six of her eight children.