That's exactly how screenwriter Misan Sagay fell into writing Belle, which is highly fictionalized: Little is known about the black woman in the painting, Dido Elizabeth Belle, who as a young girl was sent to Hampstead be raised by her great uncle, the Earl of Mansfield. In Belle, Lord and Lady Mansfield (Tom Wilkinson and Emily Watson) briefly cluck over the color of her skin before fully accepting her into their household, along with another grandniece.
The two little girls grow into young women; Dido (Gugu Mbatha-Raw) and Elizabeth (Sarah Gadon) become as close as sisters. The most compelling twist of Belle is that Lord Mansfield, in his capacity as Lord Chief Justice, is about to rule on a case that would become a fulcrum for the abolitionist movement: The 1781 Zong massacre, in which Africans on a slave ship were killed by the crew, ostensibly because there wasn't enough drinking water onboard.
There's a great story here, but Asante can't quite harness its power. Belle moves stiffly, encumbered by too many petticoats of expository dialogue. Mbatha-Raw tries to bring the right proportions of elegance and warmth to the role, but she can't bust out of the movie’s overpolished costume-drama conception.