Even though this image is from Netherlands, I uploaded it because history says, she was born in Cologne, Germany:
The Sibyls were legendary prophetesses who were said to have foretold to the Gentiles,as the prophets has foretold them to the Jews, various aspects of the Advent and Sojourn upon earth of the Messiah. They were twelve in number: the Persian Sibyl, the Libyan, the Delvic, the Erithrean, the Cumean, the Samean, the Cimmerian, the Tiburtine, the Hellespontine, the Phrygian, the European, and the Agrippine.
The majority of the Sibyls were said to have pagan roots, and as is obvious from the list, only one is mentioned to have hailed from Europe, so we can assume these were almost all women of color.
As for the woman in this painting, it is very likely she was a woman hired by Janssens to sit for the portrait, on account of her beauty and regal bearing. During this era of Netherlandish painting, commissioned paintings of Black people in lavish dress and jewelry were common, since much of the arts were funded by wealthy patrons who were very invested in international trade. The Adoration was also a very popular contemporaneous choice of subject, as depictions of incredibly wealthy traders with dark skin were often showcased to foster goodwill with traders from afar.
The Agrippine Sibyl
Netherlands (c. 1575)
Düsseldorf, Kunst Palast
Image via MedievalPOC