File:Saint Mary Of Blachernae 8938.jpg
The first church at the site of the sacred spring was commissioned by the Augusta Pulcheria between 450-453 (the year of her death) and her husband, the Emperor Marcian (450-457). The church was completed and embellished further by the Emperor Leo I (457-474), who added the Hagiasma (fountain of holy water) and the Hagion Lousma (sacred bath). Leo I also built the parecclesion of the Hagia Soros to house the holy mantle and robe of the Virgin that had been brought from Palestine to Constantinople in 473. It was then that the church was endowed with large estates.
Sources: Internetsite of the Ecumenical Patriarchate of Constantinople