The Church of Panagia Blachernai was a Byzantine church famous for its holy spring (hagiasma). It was located in the northwestern corner of Constantinople north of the Palace of Blachernai, near the Wall of Heraclius and the Golden Horn. Starting in the Early Byzantine Era, there were three important churches dedicated to the Virgin Mary Theotokos in Constantinople: the churches of Hodegon, Blachernai, and Chalkoprateia. These churches were part of the growing emphasis on the cult of the Virgin Mary, which was connected to an effort to gather her relics in the capital. As an important church and pilgrim shrine dedicate to the Theotokos, many legends and traditions developed around it. These churches were part of the growing emphasis on the cult of the Virgin Mary, which was connected to an effort to gather her relics in the capital. As an important church and pilgrim shrine dedicate to the Theotokos, many legends and traditions developed around the church.

Legend associates the establishment of all three Marian churches - Chalkoprateia, Blachernai, and Hodegetria – with Pulcheria, sister of Theodosius II and wife of Marcian. However the basilica at Blachernai is more likely the product of the emperor Justin in the early 6th century, with Leo I (457-474) and his wife Verina probably built the octagonal chapel of Hagia Soros to house the Virgin’s maphorion (veil”) after it was translated to Constantinople in 473. It, though, relied on legends about the maphorion from legends and the zōnē (“cincture”) of the Theotokos (associated with the Church of Chalcoprateia) which are difficult to disentangle. Leo I and his wife also might have been responsible for building the Hagion Louma, which included the louma (bath) included an apodyton (dressing room), a kolymbos (described as containing a basin of water), and a chapel of St. Photeinos. The basilica built by Justin I is described by Procopios describes as a three-aisled basilica, with two colonnades made of Parian marble. Justin II (565-578) restored the basilica, adding two niches to give it a cruciform plan. The basilica had a narthex and galleries accessible through a staircase. It seems that the bath was located to the south of the basilica towards the palace. 

By the 7th century, the Church of Blachernai was considered the most important house of the Virgin Mary at the capital. This is in part due to the role it was believed to have played during the Siege of Constantinople by the Avars and Persians in 626. According to tradition, during the siege the patriarch of Constantinople Sergios (610-638) walked in procession holding either the maphorion or an icon from Blachernai around the city and on its walls. After the end of the siege, the people of Constantinople gathered at the Church of Blachernai, where a thanksgiving vigil was held. During the vigil, the Akathistos Hymn was chanted in gratitude for rescuing the city – it supposedly had been composed by the Patriarch Sergios on the same day (though another tradition claims it was originally written by Romanos the Melodist in 553). After the siege, Heraclius extended the walls of the city to protect Blachernai, which originally lay outside the walls of the city.

Many other traditions are centered on the Theotokos of Blachernai as the divine protector of Constantinople. The Theotokos was said to have protected the city during the Arab sieges of 676-678 and 717-718 and the siege of the Rus in 860. This protection was often connected to the icon of the Theotokos Blachernitissa. This epitaph, though, does not refer a single icon, but rather a range iconic types from Blachernai. In 1070 the church of was destroyed by fire and was rebuilt by the emperors Romanos IV Diogenes (1068-1071) and Michael VII Doukas (1071-1078) probably following the previous plan. Following the discovery of an icon of the Theotokos Blachernitissa during this restoration work, it became associated with military victory. The icon regularly accompanied emperors on military campaigns during the 11th century. It was an iconographic variation of Virgin Orans with Christ-Child that was used on coins and seals. In this way, the hagiasma at Blachernai was overshadowed by its icon, just as the hagiasma at Hodegon was similarly eclipsed by its more famous icon.

The Palace of Blachernai was located just south of the Church of Blachernai. While the palace was in existence by 500, it became the primary imperial residence during the Komnenians era. Under the Komnenian, buildings were added to the complex and it was strongly fortified. It proximity to the church perhaps served an ideological function for the Komnenian dynasty. Following the recovery of Constantinople by Michael VIII, the palace served as the imperial residence for the Palaiologans as well. As Blachernai became the residence of emperors, the routes for imperial processions radically changed, starting from the acropolis rather than the Golden Gate. The Church of the Blachernai had occasionally been included in imperial processions even in the Early Byzantine Era. The first known procession to the Blachernai church was led by Emperor Maurice in 602. A discontent mob threw stones at him and while he managed to escape, this was part of a series of events that led to Maurice’s overthrow and violent death. According to the Book of Ceremonies, the feast of Hypapante was celebrated in the palace and church at the Blachernai. The final stage of the development of imperial processions in Constantinople under the Palaiologans celebrated ecclesiastical holidays at the palace and church of the Blachernai rather than Hagia Sophia. 
During the Latin occupation of Constantinople, Baldwin II (1228-61) used the Palace of Blachernai as his residence and the Church of Blachernai was occupied by Catholic clergy. Following the Fourth Crusade, a fragment of the cincture originally kept at the church at Chalkoprateia was supposedly kept in the Church of Blachernai. In 1434, short before the Fall of Constantinople the church burned down for the second time, after which only the hagiasma survived. While traces of it were visible still visible in the mid 16th century, soon after it completely disappeared. A large marble slab, discovered in the garden of the church, is possibly the remains of the church complex. In addition, several capitals have also been found in the area. The property was purchased in 1867 and a small shrine was built around the hagiasma. Currently a modest modern Orthodox church rests over the hagiasma.