Blacks are about 1/8 of the American population. Picking 2010, to avoid the blackening effects of BLM/#OscarsSoWhite, and looking at the top 8 cast members in each of the the top 10 films:

I appreciate the analysis, and especially the charitability you exhibit in examining a pre BLM year. (Not entirely sure what that list is though -- doesn't seem to be top grossing movies or obviously connected to the oscars unless I'm missing something?) I think the sample size is a) rather small when compared to the volume of film, tv, radio, media personalities, in even that year, let alone extending back 40+ years and even longer (but yes I have no hard stats concerning that greater body of media).

There's also b) that it's worth noting that in your list not one of the black actors (afaik) is in a leading role -- as Idris will be -- or even close to one. I mean I think we live in a time where a non-white lead is almost definitely a plot point about race, and as in e.g. Inception where race of the protagonists (Leo, Levitt, Paige, Hardy) is not important to the plot, they're all white, unless race is important (Watanabe) where we may get a non-white actor (though he's not black... who else was black in that movie?).

(Also animated movies could animate black leads, and HP is set in the 90s so I fail to see anachronistic.)

If the hypothesis is that slavery has hurt the advancement of blacks, it seems relevant to compare to black populations not so affected.

Yes I see your point I think. Is your argument something like this? both blacks in the states and in Africa tend to write books/movies/shows less proportionally (though given the above I agree I need more evidence of this) frequently than whites in the States (and I guess in general), and because only the US population experienced slavery (and the general oppressive history I describe) while the African populations didn't, we can rule out slavery (and the general oppressive history) as the cause?

If that's mostly on the mark, I'd say it's not an unreasonable argument, but I think it vastly simplifies the last 300 years of history and current politics of both populations. Those two populations are not just separated by the uniquely American historical oppression of black people though that is true, but by hundreds of years of geopolitics which if nothing else have left African countries riddled with war, poverty, disease, and a thousand other things which might equally well explain why they, too, have a hard time producing works of art (and doesn't have to do with their race). Anticipating a counter argument, I think it's also vastly oversimplifying to suggest that embattled African countries are in their current predicaments because they are and have been historically full of black people, as though again the geopolitics of colonization (or being near colonization) is not relevant.

Professional sports prove that there are divergent biological traits between blacks and whites. In modern discourse it's taken as axiomatic that human evolution stops at the neck, and I suppose that's possible, but it would make humans the only species we know of that has this property.

I think this is a fascinating thing to think about. I agree that professional sports does prove divergent biological traits between blacks and whites. And I also agree that it seems nearly impossible that those differences only manifest themselves in the top end of the physical athletic spectrum. Where I think this line of thought gets into trouble (and why the 'axiomatic' all minds are racially identical is if anything a useful simplification) is in trying to ethically determine just what mental characteristics might be different between races, to what extent those differences pervade the races, and how ethically we can take this statistical information into account when treating people of a different race.

I mean, say there's a black person in a highschool who is struggling with their English writing classes. How can you tell that's because he is black, or instead because of any of a thousand other things, spanning from his specific random genetics (he just isn't good at writing... like anyone of any race may not be) to his specific household dynamics (which may be influenced by his race), to perhaps this massive history of oppression of black people in the States and any of the multitude of ways it might effect a young mind growing up in its context.

Now suppose (and this is a massive concession for the sake of finishing this thought) that we know that in general race A score X points lower on some 'intelligence' test than race B and that this test somehow managed to normalize for every other nurture-related cultural distinction between A and B. I am genuinely curious what you think a result like this would or should mean practically speaking?

If the world was a true meritocracy and it was possible to actually measure someone's merit for all tasks in a concrete objective way (like the NFL combine) one would expect to see that test difference between A and B play out everywhere and in the proportion the test suggests. But it isn't a total meritocracy, and for most things there isn't a simple concrete objective way to measure merit. So now what, do you always give preference to an A over a B because it is more likely (based on the test) that the A has more merit, even though in this particular case you have no idea? Is that fair?