Theotókos ton Vlachernón); Turkish name: Meryem Ana Kilisesi) is an Eastern Orthodox church in Mustafa Paşa Bostanı Sokak in Ayvansaray in the Fatih district of Istanbul, just inside the old walled city.

In 450, Empress Aelia Pulcheria began to construct a church near an ayazma or holy water spring situated outside the walls of Theodosius II at the foot of the sixth hill of Constantinople.

[7] When the victorious Heraclius returned to Constantinople, bringing back the True Cross which had been captured by the Persians in Jerusalem, the Patriarch received him at Saint Mary's.

In 926 during the war against Simeon of Bulgaria the potency of the relics of the Virgin also helped convince the Bulgarian Tsar to negotiate with the Byzantines rather than assault the city.

[8] As a consequence of that decision, Emperor Constantine V ordered its figurative mosaics to be destroyed, and replaced them with others representing natural scenes with trees, birds and animals.

[2] In 843, with the end of Iconoclasm, the Feast of Orthodoxy was celebrated for the first time in the church of Blachernae with an Agrypnía ("holy Vigil"), which occurred on the first Sunday of Lent.

[8] Thomais of Lesbos, a laywoman who was later considered a saint, frequented the church regularly in the 10th century and prayed before the Ayía Sorós (where lay people were not allowed).

[9] The Blachernitissa was discovered again during restoration works executed during the reign of Romanos III Argyros in the early 11th century[12] and once again became one of Constantinople's most venerated icons.

The Church of Saint Mary was completely destroyed during a fire in 1070, but was rebuilt by Romanos IV Diogenes and Michael VII Doukas to the same plan.

The religious complex of Blachernae comprised three buildings: The Church of Saint Mary, the Chapel of the reliquary (Ayía Sorós), and the Sacred Bath (´Ayion Loúsma).

[11] Justinian appears to have added a dome to the church, since Procopius, in his work De Edificiis, says that both colonnades curved in the middle of the nave to describe a semicircle.

The Spanish ambassador Ruy Gonzáles de Clavijo, who visited Constantinople in 1402, wrote that the building was divided into three aisles, with the central one higher than the flanking ones.

[17] Each year, on August 15 (the feast of the Dormition), after the adoration of the Mafórion (holy veil) of the Virgin, the Emperor plunged three times into this sacred pool.

[17] The small church which today encloses the ayazma (holy spring) has a trapezoidal plan with a sloping roof, and is adorned with modern icons and frescoes.

The holy spring, which is believed to have healing powers, is still a popular destination for Orthodox and Muslim pilgrims, who toss coins and hairpins into the pool.