I do think that Marius has to be white, blond, and blue-eyed. For Armand's story, he has to look a bit like Lestat. Because I feel like the fact that Lestat looks like Marius is the reason Armand gets obsessed with him in the books. And it makes sense for that to be the reason for Armand on the show, too. At least to me, more sense than Lestat is just so special that everyone gets obsessed with him. Also, I'd be disappointed if they tone down Marius's philosophy being very Eurocentric and having a tinge of white supremacy. And I like the idea that Marius's fondness for Lestat is at least partially motivated by ego. To me, Marius is like one of those old white male academics, the ones who think civilisation peaked in Ancient Rome and that look down on every other corner of the world as less.
Mm, I do hear you, and I don't disagree overall, but I actually don't think it's super important that Lestat looks like Marius necessarily? There should be a connection, of course, but I think there's a lot of room in the show to have Lestat's immediate appeal to Armand be tied intrinsically to art, especially given Armand himself has been whitewashed in the Vecchio painting in 2.04 already which goes to your comment on the Eurocentrism/white supremacy angle already. Lestat is an ideal of beauty in Italian art from the era Armand would've been with Marius - everyone rightfully compares Lestat to Caravaggio's boys and Michelangelo's David, but you could pretty easily compare him to Antonio del Pollaiuolo's Apollo, Botticelli's Venus or Raphael or Lippi's Madonnas too - and I can see Armand seeing Lestat in a certain light because he looks like the angelic figure of the paintings Marius did or collected. In a lot of ways, that could elevate him even moreso in Armand's mind, particularly after the chapter of abuse and squalor with the Children of Darkness.
But yeah, Marius' Eurocentrism should be a factor, he is from Ancient Rome, and has lived in Italy for an extended period of time, but there's a lot of evidence that Ancient Rome was pretty diverse. There's a great article from The University of Warwick on it here, but Ancient Rome's trade routes with North Africa and the Euphrates, which passes through Iraq, Syria and Turkey, were really strong, and there's a lot of evidence to suggest there was a fair amount of migration. In that sense, I think it would be pretty easy to position him as having been born in Rome to migrant parents and having his cultural connections being still rooted entirely in Rome / Eurocentrism without him necessarily being white.
Again, I don't disagree with you though, and kind of do feel the show will probably cast a white (although maybe Meditteranean-white) actor in the role.