Born in Santarém, when Blanche was two years old, her father gave her, as a perpetual donation, the city of Montemor-o-Velho with the condition that it would revert to the crown upon her death or in the event that she married outside of Portugal. Following the footsteps of her great-aunt, queen Theresa of Portugal, Queen of León and other female members of the royal house and nobility, she went to live at the Monastery of Lorvão in 1277 although at that time she was not a nun and was never the abbess of this religious establishment.