According to some legends, the Black Madonnas do not depict Mary, the mother of Jesus, but rather Mary Magdalene, whom some believe was the wife of Jesus. It is certainly true that where there are Black Madonnas, there generally is also a strong tradition of venerating Mary Magdalene.
Those who actively promulgate this view, also claim that the Pieta of Michelangelo is not supposed to be of Jesus and his mother but Jesus and the Magdalene analogously to the mystery-gods Isis and Osiris. According to them the Pieta which Michelangelo intended to be on his tomb illustrates this clearly.
One may interpret it showing the love of Mary Magdalene for Jesus, the small person on the side being their child. This interpretation of the Black Madonnas is also connected to the fringe belief that the descendants of Jesus and Mary Magdalene are living today.
Besides a reinterpretation of a long list of legends, the Black Madonna is claimed to symbolize the hidden story of Mary Magdalene, after she fled to Egypt pregnant with Jesus's child and then went on safely to Provence, France.
A reasonable claim is that she was venerated by many "Heretics" who claimed private knowledge of the "true story" of "what happened". They would have had to keep such knowledge hidden because presumably men in power through apostolic succession would be jealous of a Mary Magdalene who would be considered the "disciple Jesus loved best".