> in the history of England, the United Kingdom and the whole Commonwealth there has not been a single instance of a king or queen being anything other than - here comes the dreaded word - white.
This is, in fact, (while completely beside the point) wrong. There are several non-white Kings and Queens in the history of the Commonwealth; e.g., Mswati III of the Kingdom of Eswatini.
> what's the point of making them black?
What's the point of alternate history and historically-inspired fantasy? That's a kind of big question. Do you literally think every film or show should simply depict life literally exactly how it actually occurred at some time in the past?
> in the US, I guess you could make Lincoln, Washington and Reagan black.
Or make Lincoln a vampire hunter.
(Of course, there isn't a long history of rumors that Lincoln actually was a vampire hunter taking as their starting point a too-literal reading of contemporary descriptions probably meant as throw-away insults.)
The complaints about Bridgerton seem to be highly-selective blindness to the entire concept that overtly fictional entertainment is typically something other than an exact recreation of history. (And/or assertion that race is somehow a uniquely unacceptable thing to fictionalize for unspecified reasons.)