Dido Belle, the mixed-race daughter of an 18th-century British aristocrat, is the subject of a mysterious painting and a new film, Belle. Here we unravel her story – and the puzzle of her pose

The enigma of Mona Lisa's smile? Who cares? The mystery of Dido Belle is much more intriguing. The double portrait Dido Elizabeth Belle and Lady Elizabeth Murray, once attributed to Johann Zoffany and now hanging in Scone Palace in Perth, depicts two elegant 18th-century women in silks and pearls at Kenwood House in London. Beyond them, you can just glimpse St Paul's and the rest of the Georgian cityscape. Nothing unusual about any of that, but for one detail – Dido is mixed race.

The first question that strikes the viewer is: why is Dido pointing at her cheek? It is a "puzzling gesture", wrote English literature lecturer Christine Kenyon Jones, in an article for the Jane Austen Society of North America. "Is it meant to draw attention to her skin colour, or simply to her smile and her dimples?" Might it even be, as a new theory suggests, an allusion to the Hindu deity Krishna?

These questions – and the mystery of what she was doing, both at Kenwood and in the painting – feel especially topical since Dido, the daughter of a former slave and a British aristocrat, is now the subject of a film, Belle. Meanwhile, a new biography of her great uncle and benefactor Lord Mansfield sheds more light on her 30 years at Kenwood. The painting has even inspired a novel. "The reason it struck me," says Caitlin Davies, author of Family Likeness, "was that I grew up near Kenwood, so had been in and out of the house for 45 years. Then suddenly, in 2007, I saw this portrait when it was on temporary display, in an exhibition, Slavery and Justice."

Davies, who is white, had taken her mixed-race daughter along. "She was struck still by it, because pretty much all the other portraits on the walls were of white aristocrats – and here was someone who looked like her." Her novel is about a mixed-race girl growing up in a small Kent town in the 1950s, ostracised because of her colour.

Amma Asante, the director of Belle, explains why the painting inspired her film: "You see a biracial girl, a woman of colour, who's depicted slightly higher than her white counterpart. She's staring directly out, with a very confident eye. This painting flipped tradition and everything the 18th century told us about portraiture. What I saw was an opportunity to tell a story that would combine art history and politics."

And Dido's life story is an irresistible subject. She was the illegitimate daughter of Lord Mansfield's nephew, Sir John Lindsay, a navy captain, and Maria Belle, an African woman (possibly a former slave) whom he captured from a Spanish vessel in the Caribbean. Dido was sent to England as a child by Lindsay and, from the 1760s, was brought up at Kenwood House by the childless Lord and Lady Mansfield, along with her cousin, Lady Elizabeth Murray, whose mother had died.

"It is not known if Dido was willingly parted from her mother," wrote historian Gene Adams in her 1984 paper Dido Elizabeth Belle: A Black Girl at Kenwood, "but materially speaking, it would certainly have helped both mother and child." According to English Heritage, which runs Kenwood, "her position in the household may have been that of a loved but poor relation and she did not always dine with guests".

But was there more to it than that? Lord Mansfield was Britain's most powerful judge and, as Lord Chief Justice in 1772, he presided over the landmark case of a runaway slave called James Somerset. He ruled that a master could not take a slave out of Britain by force, a judgment seen as a key stage in the eventual abolition of the slave trade. "Slavery," he said in his judgment, "is so odious that nothing can be suffered to support it."

There were those in Georgian England who argued that Dido influenced her great uncle's decision. Francis Hutchinson, an American living in London, wrote of his visit to Kenwood: "A Black came in after dinner and sat with the ladies and, after coffee, walked with the company in the gardens, one of the young ladies having her arm within the other … He calls her Dido, which I suppose is all the name she has. He knows he has been reproached for showing fondness for her – I dare say not criminal."

He went on to mention the Somerset case: "A few years ago, there was a cause before his Lordship bro't by a Black for recovery of his liberty. A Jamaica planter being asked what judgment his Ldship would give? 'No doubt,' he answered. 'He will be set free, for Lord Mansfield keeps a Black in his house which governs him and the whole family.'"

In any case, was Lord Mansfield really an opponent of slavery? "He was very reluctant to annoy the slave owners and vested interests," says Norman Poser, author of Lord Mansfield, Justice in the Age of Reason. "He rather hoped things would just go on as they were.'" And while Mansfield was clearly fond of Dido, Poser says there is nothing to suggest she changed his views.

But back to the painting. Why does Dido look as if she's rushing past her cousin on an errand? For Davies, one possibility is that this started as a single portrait. "It looks like the portrait of Elizabeth came first and then someone wanted the two young women together, so Dido was added. The touch between them can seem awkward – is Elizabeth pushing her away? But perhaps the painter just kept Elizabeth as she was, with one arm held out."

Film of the painting … Belle. Photograph: Allstar/Sportsphoto

In the film, Dido's odd status in this white, stratified society is made plain. "I am too high to eat with the servants, too low to join you at dinner," Dido, played by Gugu Mbatha-Raw, tells Tom Wilkinson as the Earl of Mansfield. But that isn't the whole story: after Lady Mansfield died, Dido stayed on to care for her uncle, reading newspapers to him at breakfast as he succumbed to rheumatism. He died, aged 88, in 1793, providing for Dido in his will, as well as setting aside a substantial sum in a codicil. She married, left Kenwood and had three sons, dying in her early 40s in 1800.

In any case, there's more going on in the depiction of Dido than an image of an uprooted, lonely girl. Mario Valdes, a US historian of the African diaspora, suggests that her turban may be part of an attempt to Indianise Dido. Between 1770 and 1771, he points out, her father served as His Britannic Majesty's Minister Plenipotentiary in India. What does that have to do with Dido's gesture? "One interpretation is that she is pointing to the difference in complexion between herself and her cousin," says Valdes. "But I would argue that a far more sophisticated approach is at play."

There is a sculpture that shows Krishna in a similar pose, Valdes explains, and a story that he was once slapped by a female deity for taking on the appearance of her sister and her husband. When this sister tried to console him, he smiled, pointed to his bruised cheek, and exclaimed: "She has shown that all three of us are one and the same." Valdes says: "What Dido's pose apparently proclaims, therefore, is that she and her cousin share the same humanity and innate worthiness."

Archivists at Scone have found nothing to corroborate the account, but that would be hard at nearly 250 years' remove, not least because the painting has had the Zoffany attribution withdrawn. If we can't find out who painted Dido, we can't know what the artist meant in his world.

There's one last question worth thinking about. "I wondered if Dido herself ever saw the painting," says Davies. "I was intrigued by what she might have thought of it. But I've been told it wasn't shown at Kenwood during her lifetime."

Perhaps she would have liked it. Others certainly do. At the Scone Palace gift shop, alongside reproductions of the double portrait, you can buy cropped images showing Dido, alone and smiling. It's an image that's been printed on to pocket mirrors, key rings, and magnetic notepads. It seems Dido Belle, for all the obscurity of her later life, is in 2014 something of an icon.

• Belle is on general release on 13 June.

• Amma Asante: I'm bi-cultural

• Slave's daughter who lived in Georgian elegance