The most widely accepted explanation is that the Madonnas were not meant to represent a black-skinned Mary, but rather they are black simply because of the wood or stone they were carved from, or that smoke from candles and incense have turned them black. In many European languages, especially prior to the pre-modern age, the word "black" rarely directly meant "person of African race, African descent," but merely "dark in color."

According to some legends, however, the Black Madonnas do not depict Mary, the mother of Jesus, but rather Mary Magdalene. It is certainly true that where there are Black Madonnas, there generally is also a strong tradition of venerating Mary Magdalene. According to this view, the Black Madonna is claimed to symbolize the allegedly hidden story of Mary Magdalene, after she fled to Egypt pregnant with Jesus's child and then went on safely to Provence, France.

Yet another explanation posits that the figure represents a Christianised version of the goddess Isis with her child Horus.

In some countries and towns, however, black madonnas exist because most of the believers are of dark skin or probably the sculptor/artist was of dark origin himself/herself. This is not the case of Germany or Switzerland.

http://en.wikipedia.org/wiki/Black_Madonna

Historically, alabaster "white" madonnas are far newer than their African counterparts. White Madonnas only began appearing during the Middle Ages, around the time the Marian cult finally gained approval with the Catholic Church.

http://www.africanbynature.com/links/madonnalinks.html