When the group of Mercy monks under the guidance of St. Pedro Pascual was planning to open its first house in the area of Jerez, the city council asigned them a field outside the city walls where they could build. It was known that there had been a tile and brick manufactory in that place, of which some parts of the oven still stood. When the workers were demolishing the remaining ruins in order to build the new foundations, they found a cavity with a small niche containing a beautiful image of the Blessed Virgin all blackened, no doubt by the smoke of the tile burner. When the discovery was reported to St. Pedro Pascual he was very pleased. He declared it to be manifest proof that the Queen of Heaven would protect the futue monastery and the city. The Black Madonna was worshipped in a temporary location until the new monastery was built and she could be installed in her place of honor behind the main altar. Nobody doubted that she should be known as Our Lady of Mercy and be the celestial head of the order she had come to protect and lead. According to this version she was declared patroness of Jerez already in 1272 and the city swore her loyalty in 1300.(*1)

The Black Madonna placed behind the altar as if she was presiding at mass

The other opinion claims that Our Lady of Mercy didn’t come to Jerez until the 14th century.(*2) It recounts that in 1340 Christian forces reconquered the town of Algeciras from the Muslim occupiers, who had held it for a few hundred years. They converted a mosque into a Christian church and installed in it the Virgin of the Palm Tree. The Virgen de la Palma, as the locals call her, is another Mary statue that had been discovered under somewhat mysterious circumstances. To this day she is celebrated as the patroness of Algeciras. The Christian dominion of the town didn’t last long. During the second half of the 14th century, Muslim forces in their turn reconquered Algeciras without much difficulty. Many inhabitants fled as soon as they saw the Muslims approaching, taking with them a few belongins. Among these was a soldier, whose name is unknown. Anticipating the sure desecration of the sacred image of the Virgin of Mercy, he bore her off to the city of Jerez, where he arrived after a few days. There he presented himself at the Mercy monastery and explained the reason for his journey with so precious a cargo. He asked the Prior to grant asylum to the sacred image, until he could hopefully return for her one day and bring her back to her place of origin. The monks agreed and placed the Black Madonna temporarily on a pedestal in the sacristy of their church, expecting the eventual return of the soldier. But during the first night, the friars were awakened by neighbors who had seen a great light shining from the window of the sacristy. The monks ran, thinking that perhaps a candle had set their church on fire, but when they entered the sacristy they discovered in awe that the glow, which flooded it, came from the Madonna.

This phenomenon was repeated for several nights in a row. So the monks held a meeting and decided that it was a clear sign of the desire of the Blessed Virgin Mary to be venerated in their church. Their reasoning seemed to be confirmed by the fact that they never again heard of the soldier who had brought her to them. So she was placed center stage behind the main altar and given the title Our Lady of Mercy, indicating that from now on she was the head of the order.
According to I.E.S. Francisco Romero Vargas, both opinions are backed up by old manuscripts in the Mercy order archives of the city. Yet it seems to me that the second story actually belongs all together to the Virgin of the Palm Tree with whom it starts, never explaining why it switches to the Virgin of Mercy mid way. The patroness of Algeciras indeed was taken from the city during the Muslim occupation of 1369 and didn’t return until the 18th century.(*3) Looks to me like she spent 400 years with the Mercy monks.

What all agree upon is that the cult of the Black Madonna of Jerez received a great boost in 1599 to 1600. There was a harrowing drought and the monks recommended that the statue be taken on a procession as a way of imploring the Queen of Heaven for help. It must have helped, because shortly thereafter the city council voted to declare her patroness of Jerez and for years to come Lady Mercy was honored on April 30th, the day the rains came. From then on her shrine was filled with votive offerings and jewelry. Later her feast day was moved to August 15th, the day of Mary’s assumption into Heaven. Now she is honored with a procession and festivities on September 24th.
Until the 19th century Mother Mercy was always in the care of her monks, but during that century she had to leave the monastery on three occasions of political anti-Catholic unrest in the wake of the French Revolution. Twice her monastery was converted into a hospital.
Finally at least the church, though not yet the monastery, could be reopened. It was often run by priests, who had been monks of Mercy but forced to disband their order. It wasn’t until 1940 that their community was reestablished as the custodians of the shrine.

This Black Madonna wasn’t canonically crowned until May 31st 1960, but the people recognized her as their queen long beore that.
Little is left of the original statue. Because the custom of dressing images of Mary up like royalty made people cut up old statues so that they could be treated like a puppet. The audacity to cut them into pieces for this purpose began in the 17th century.

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*1: “La Merced” in on-line journal “Jerez Hoy 2020"
*2: Presented in I.E.S. Francisco Romero Vargas’ article “Nuestra Señora de la Merced. Patrona de Jerez”
*3: See the “Fiestas de Algeciras” article on Wikipedia.