Claims of African ancestry are attributed to which Queen of Great Britain?

In a 2009 episode of the PBS TV magazine, Frontline, Mario de Valdes y Cocom claimed that Charlotte may have had African ancestry and speculated that Scottish painter Allan Ramsay emphasized the Queen's alleged "mulatto" appearance in his portrait of her to support the anti-slave trade movement.

Valdes incorrectly said that an early-19th-century medical practitioner, Baron Stockmar, was Queen Charlotte's personal physician and that he had described the Queen as having a "mulatto face" in his autobiography. In fact, Christian Friedrich Freiherr von Stockmar was personal physician not to Queen Charlotte, but to Prince Leopold of Saxe-Coburg-Gotha in 1816, at the time of Leopold's marriage to Princess Charlotte of the United Kingdom.

On the PBS website page for this episode, Valdes interprets an excerpt of a poem reproduced there as attesting to Charlotte's African features, but it clearly refers to her as descended from the Vandals, an East Germanic tribe. According to Valdes, Queen Charlotte's African contribution could have been inherited three to six times over from one ancestor nine generations removed, Margarida de Castro e Sousa, a 15th-century Portuguese noblewoman, who traced her ancestry to King Afonso III of Portugal (1210–1279) and one of his mistresses, Madragana (c. 1230–?).