The thing that started this trend, I think, was the Broadway production Jesus Christ, Superstar in 1971. It had Ben Vereen as Judas. On the album, Murray Head, a white guy, had that part, so putting a black performer in his place, in a part generally thought of as “white” was a pretty bold move.
(Pearl Bailey had played Dolly Levi in Hello Dolly on Broadway in 1967 – but they had the entire cast switched to black performers. Vereen was, aside from chorus and dancers, the only black performer in the cast AFAICR)
the next year Vereen appeared in another Broadway production – Pippin – which was very (VERY) loosely based on the life of Pepin the Short, son of Charlemagne. The play was blatantly anachronistic and ahistorical, and Vereen was given the part of the “Leading Player” of a performing troupe telling the life of Pippin. Again, having a black performer in the part was bold and unexpected, but a.) Vereeen was spectacularly talented and b.) it was another opportunity to expand the parts a black actor could inhabit. When others played the part, they were black actors, too (when I saw the show Vereen’s part was taken by Northern Calloway from Sesame Street). One reason was that they played off the character. When he is among a group of revolutionaries (lead by Pippin), the other revolutionaries are dressed in dark cloaks, like Sneak, Snoop, and Snitch from the cartoon Gulliver’s Travels, but Vereen’s character was in a black leather jacket and shades, like a Black Power figure.
After that black actors started getting snuck into a lot of roles where the characters were traditionally white, generally without any attempt to explain or rationalize things. Whoopi Goldberg played Pseudolus in A Funny Thing Happened on the Way to the Forum (a part previously played by a white guy. Phil Silvers played the part with anachronistic glasses). They put Morgan Freeman in as a black character in Kevin Costner’s Robin Hood, Prince of Thieves, where he was arguably the best character there. (There was a sliver of justification, in that Robin was returning from the Crusades, and Freeman’s character was a Moor) This started a new tradition – Robin Hood films started including black characters, something they hadn’t done before.
And Hamilton really expanded the concept, using lots of diverse actors in its cast. The 2019 movie The Personal History of David Copperfield was all over the map, as far as diversity went, with Dev Patel in the lead (of Indian ancestry, although his parents were from Kenya and he was born in Britain), Benedict Wong as Mr. Wickfield, Nikki Amuka-Bird as Mrs. Steerforth, and others.
So Bridgerton carried it further with even more roles being filled without regard to background, race, or ethnicity. It was pretty clear to me what they were doing even without explanation.