I wouldn’t say it was a ‘stunt’; it was more a case of the film’s director, Josie Rourke, trying to bring the casting practices of British classical theatre into the world of film. She comes from a hugely successful theatre background (particularly as artistic director of the innovative London space, the Donmar Warehouse), and so-called colour-blind casting is completely normal in that sphere. Adrian Lester (who played the fictitious character OP mentions in ‘Mary Queen of Scots’, for example, has played Hamlet on the London stage, and received great acclaim for his Henry V at the National Theatre. It is a (welcome) fact of life in classical stage plays in the UK to have actors of any background being given the opportunity to play the widest range of roles.

As for the historical reality of black people in sixteenth-century England, Miranda Kaufmann’s ‘Black Tudors: The Untold Story’ (2017) offers a very good overview. She finds evidence for around 360 black individuals in England from 1500-1640, from many origins and walks of life, including a royal trumpeter, John Blanke.

In the diplomatic sphere, we also have the interesting figure of the Moroccan ambassador Al-Annuri, who visited Queen Elizabeth’s court in 1600, and was the subject of an intense and dignified portrait, showing how the racial/religious ‘other’ could (artistically at least) be viewed with respect and admiration. Many think that he was an inspiration for Shakespeare’s Othello.

However, the year after Al-Annuri’s mission, we have a draft proclamation from the Queen ordering the expulsion of all ‘negroes and blackamoors’ in the kingdom, suggesting a systemic political racism at work (the last years of Elizabethan’s reign were characterised by political and socio-economic turbulence, and scapegoats are far too often sought during such periods). Whether the command was ever promulgated is contested, however.

Some Europeans courts did have African servants. The best known example is John Blanke, a black musician who went to England with Catherine of Aragon and later worked for Henry VIII, becoming a relatively well-known member of Henry's entourage.

However, all the titled nobility and virtually all of the servants in European courts were European.

Why does the movie present something different? The makeup of Mary and Elizabeth's courts was a deliberate and explicit artistic choice on the part of director Josie Rourke. Rourke has explicitly said as much:

“I was really clear, I would not direct an all-white period drama. Adrian [Lester], who plays, Lord Randolph, grew up 40 miles from the birthplace of William Shakespeare; he is one of our eminent Shakespearean actors. I needed to cast an ambassador who could move between the two courts and help this make sense. I don’t understand why you wouldn’t cast him.”

Rourke's position lead to some casting choices where the courts of Mary and Elizabeth look less racially homogenous than they actually were. Of course" Mary and Elizabeth's courts (Mary's in particular) were quite diverse in terms of nationalities - there were courtiers from all over Europe.

As a result of Rourke's choices, many actors in the film don't resemble historical people they're playing (not that this is admittedly a new phenomenon in Hollywood!)

The ambassador "Lord Randolph" Rourke mentioned earlier is based on Thomas Randolph, one of Queen Elizabeth's secret agents and ambassadors. As far as I know there's no known portraits of him - however, he wasn't of African origin or even mixed-race.

Adrian Lester as "Lord Randolph"

Bess of Hardwick (aka Elizabeth Cavendish, aka Elizabeth Talbot, Countess of Shrewsbury) was an English courtier and noblewoman. She's played by a Briton of Chinese ancestry, Gemma Chan.

Gemma Chan as Bess of Hardwick

The real Bess of Hardwick

One of Mary's companions, Lady Mary Seton, was born to a Scottish father and a French mother.

Izuka Hoyle's portrayal of Lady Mary Seton

The real Lady Mary Seton