The 35.56 carat diamond - with a 300-year-old royal history charting the flow of power across Europe - had been expected to earn bids of up to nine million pounds at the Christie's auction.
But it has now broken the world record for any diamond sold at auction. The stone was purchased by Bond Street jeweller Laurence Graff.
The Wittelsbach diamond, which is almost an inch across, was given to Infanta Margarita Teresa by her father King Philip IV of Spain on her engagement to her uncle Leopold I of Austria. It was later owned by the Wittelsbach family of Bavaria.
The infanta, a beautiful child with blue eyes and blonde hair, was the central figure in Las Meninas (1656), the most famous painting by Spaniard Diego Velázquez.
It depicts the girl, her maids of honour and members of her family and even the artist himself curiously looking out of the painting as if on the viewers themselves.
Upon her politically-motivated engagement to Leopold I, who later became the Holy Roman Emperor, her father gave his 13-year-old daughter the diamond as part of the dowry.
She died aged only 21, having given birth to four child, of which only one survived beyond childhood.
The diamond, reputed to have originated from the Indian diamond mines, passed into the Austrian and then Bavarian crown jewels, before being sold into private hands in 1931.
It has been in the current private collection since 1964.
A Christie's spokesman said "no examples" of coloured diamonds comparable to the Wittelsbach had been ever been offered at auction before.
She said: "Blue diamonds are rare and to offer a blue diamond of this size, quality, shape and provenance is truly extraordinary."
The nearest comparable coloured diamond sold at Christie's was one just a third of the size, a 13.39 carat intense blue diamond that fetched $8.9 million (£5.5 million) in May.
François Curiel, chairman of Christie's Europe and its international head of jewellery, said: "It is a great honour and a lifetime dream to handle a museum quality stone such as the Wittelsbach.
"The appearance of a large blue diamond, among the rarest of colours, with a history that can be traced back to the 17th century and 300 years of royal connections will surely be a thrilling occasion for all collectors of exceedingly rare jewels and works of art."
