"Númenor was thrown down and swallowed in the Sea, and the Undying Lands were removed for ever from the circles of the world. So ended the glory of Númenor."
That's directly from the appendix to RotK, so yeah it was described in the source material that Amazon is using.
Their obvious variations in phenotype is as much an indication of "mixed race" as dark skin vs light skin. And yes, while much of House Beor was wiped out, not all were and all it takes is one member with the specific genetic mutations for the physical traits of House Beor to continue passing down through generations. Obviously those traits for darker skin and hair were pretty strong since Aragorn's description is more in line with the phenotype of House Beor than of House Hador.
Speaking of Aragorn, were you just as livid when Erendil, Isildur, and Aragorn were portrayed in the movies as being bearded when their elvish blood should have prevented that? How many forums threads were spawned from that divergence from lore?
Unlikely? Sure. But there is precedence as far back as Tar-Aldarion of kings who married below their status. He wed Erendis who was of Beorian descent, and had a shorter lifespan and was of lesser status than Aldarion since she didn't come from the royal house. Their daughter, Tar-Ancalime became the first ruling queen and due to her lineage the genetics of House Beor could continue through the line of Kings (meaning that any of those marriages to women we'll never know about could have produced children of darker skin than the typical Numenorean descended from House Hador, which in turn answers your question of how Miriel in the show could be dark skinned).
And lets be honest here, on the scale of human skin tone Addai-Robinson's is a pretty middle-of-the-road light brown, not black like the people of Far Hadar. I remember watching her on Spartacus (not a big fan of her character there. but whatever) and she didn't stand out much at all from the pretty tan, white Australian and New Zealand actors of the show. Continually talking about her like she's a fucking 10ft tall blue smurf in the midst of a bunch of pasty Englishmen is poor form.
It doesn't. I mean, maybe to people who see everyone who isn't white as "black" it might seem that way, but Ismael Cruz Cordova is Puerto Rican (a country that is predominately multiracial Hispanic) and Nazanin Boniadi is of Iranian descent.