So, as I've written in an answer to a recent question, Bridgerton is not attempting to be an accurate piece of historical fiction. It is an adaptation of a series of Regency romance novels, and Regency romance is a subgenre that is based much more on the fiction of Georgette Heyer than the actual real-world England in the Regency period. While Heyer included a great deal of detail from in-depth research, such as the inclusion of real merchants and establishments like Gunter's Tea Shop, she didn't have the opportunity to take inspiration from the work in social history that was published in the late twentieth century (her heyday was the 1930s-1960s), and so her books contain a lot of subtler errors in terms of how characters relate to each other. Those who followed her in the subgenre have continued these issues, so we see things like Daphne and Eloise being totally innocent about sex before marriage - a reasonable assumption in the 1950s, considering contemporary conservative mores and those of the Edwardian era of Heyer's youth, but not actually likely in the 1810s. And to some extent, the people who enjoy the genre know that it's a few generations removed from historical reality! As a result, the show has the freedom to take a lot of liberties (just look at the costuming).

There are some people who claim that Queen Charlotte was Black, yes. Sex and Race's (1940) chapter on "Race-Mixing in the British Isles" describes a famous portrait of the queen as "clearly show[ing] a Negro strain" - people have been tossing the idea around for a long time. However, the logic is pretty specious. Her ancestry is said to go back to Afonso III of Portugal (1210-1279) and his mistress, Madragana, who may have been a Moorish woman but may also have been a non-Moorish woman from Moor-controlled southern Portugal. Even assuming that Madragana was of Moorish descent (and therefore most likely North African), there are fifteen generations between herself and Charlotte of Mecklenburg-Strelitz. To believe that it's relevant to Charlotte's ethnicity is to engage in one-drop-rule thinking. Yes, the diary of Baron Stockmar (physician to Charlotte's son-in-law) referred to her as having a "true Mulatto face", but, well, someone can describe another person's appearance in a racist way without the other person actually being or another ethnicity. Have you ever seen the John Mulaney bit describing how other kids thought he "looked Chinese" when he was little? It's somewhat ironic that this discourse is going around today, when we've had multiple high-profile race-faking scandals; if Rachel Dolezal had said she had a single ancestor fifteen generations back who she believed to possibly be Moorish to justify presenting herself as a Black woman and NAACP spokesperson, she would probably have been dragged even harder.

The showrunner, Chris Van Dusen, was aware of this whole business, and deliberately chose to explore the possibility of Charlotte being actually, recognizably, culturally Black (referring to this as "fantasy"). This was incorporated into the script, with Lady Danbury discussing how the marriage of the king of England to a Black woman affected Black people in that country. This created some interesting characterization moments, which I think made it worth it - not to mention the empowering portrayal of the most important and powerful woman in the universe of the show as a self-assured Black woman.