It’s a subtle point, easily missed. Meghan Markle, Prince Harry’s apparent new love, is a “glamorous brunette”, “a departure from Prince Harry’s usual type” and “not in the society blonde style of previous girlfriends”, according to the Daily Mail. I think what they are trying to say is that Markle, actor, global development ambassador and lifestyle blogger, is black.

Or, to be more precise, Markle is what Americans call “biracial”. The actor, who grew up in LA but lives in Toronto, has spoken extensively about her dual heritage – her father white and a mother she describes as “100% black” – and her quest for her identity over the years.

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The British press have a different approach. They have made it clear that her relationship with Harry is scandalous, for a number of reasons: she is divorced; she is older (Markle is 35, Harry 32); she’s played raunchy scenes in the US TV series Suits – and her mother is visibly black, with dreadlocks.

These details would be unremarkable – irrelevant, inappropriate, even – were it any other celebrity romance. But Prince Harry, fifth in line to the British throne, is a different matter. The very concept of the royal family is the antithesis of diversity. The terminology says it all: “blue blood”, from the Spanish sagre azul, coined in the late 1500s to distinguish between the racially superior white Christian nobility (with pale skin revealing blue veins) and the Jews, Muslims and West Africans whom Europeans were increasingly ousting from their continent.

In spite of its roots, we are apparently still perfectly comfortable using the phrase, even though royal blood has probably been mixed for centuries. There have been Africans throughout Europe since at least Roman times, and marriages between European royals, with their fondness for black servants, slaves and extramarital reproduction, make it unsurprising that Queen Charlotte, wife of George III – described, in an era when slaves were omnipresent, as “ugly”, with a dark complexion and flared nostrils – may well have had some African heritage. So might Queen Philippa, wife of Edward III, described as having broad nostrils and a wide mouth, and as being “brown of skin all over”.

We are as unwilling to embrace the ethnic heritage of the royal family as we are to investigate their centuries-old links with the slave trade. When asked about Queen Charlotte’s origins a few years ago, the royal historian Hugo Vickers assured Brits that even if African blood had penetrated the royal bloodline “there would be no shame attached to it” and “it certainly wouldn’t show that they are significantly black”. What a relief.

As for the Markles of this world – black people who are linked to the aristocracy in modern times – there is some evidence of what awaits. Emma McQuiston, the mixed-heritage socialite who married the heir to the 16th-century Longleat estate, Viscount Weymouth, faced blatant racism, with the viscount’s mother allegedly asking him: “Are you sure about what you’re doing to 400 years of bloodline?” McQuiston told Tatler she faced prejudice on multiple fronts. “There’s class,” she said, “and then there’s the racial thing.”

Other “aristoblacks”, as some call them, have had their share of grief. Nimmy March, adopted daughter of the Duke and Duchess of Richmond, has spoken about the discrimination she faced as a child. And I’ve heard of the MP Chuka Umunna, whose maternal grandfather was Sir Helenus Milmo QC, prosecutor at Nuremberg, being dismissed disparagingly for embracing a black identity when he is descended from posh English stock. The two, in the British imagination, are often still seen as mutually exclusive.

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Who knows whether it’s intentional racism or just ignorance, but our society’s most privileged members seem to be sufficiently lacking in education that Prince Philip could liken the Nigerian president’s traditional costume to pyjamas (“You look like you’re ready for bed,” he told Olusegun Obasanjo); and Prince Harry could, unforgettably, don a Nazi outfit to a fancy dress party.

If Harry was previously oblivious to the complex world of race and identity, he’s about to get a crash course. News of his latest relationship threatens to bring Britain’s simmering, unresolved issues with the myth of royal racial purity into the open. Markle may have found peace in the grey spaces of mixed identities, and has spoken of both her positive experience of blackness and the negative – seeing her mother called the N-word, and being passed over for acting roles by an industry that regarded her as too black to play a white role and too white to play a black one.

Markle is proud of her heritage. And that’s the real difference between her and the royals – a family who have ignored questions about their history, swept them under the carpet and, frankly, hoped no one would notice.