Make a comparison with the US porn industry is a bit tricky, and maybe a bit unfair. We are comparing a country were people of color can be up to 40% of the given population in a certain area, with probably one of the most ethnically homogeneous country in the world.
So while I can agree with a representation problem in the US, it becomes a bit tricky to apply the same standard when speaking of a country with a completely different story and population.
You should also consider that a country like Japan, but also others, are completely cut out from the actual US conversation on race related issues, due to language barriers. So we can probably cut some slack to the JAV industry on that.
Regarding the choice of Black Talents, It is not that the pool of Male Talent in the US is much bigger, if you look closely you notice that the majority of the male talent has been more or less the same for the last 10 years with a few talent replacing the spot left open by the one retired (It is a hard and bad paid job at the end of the day). For the reasons described above I would guess that find new talents from African or European descent is not that easy in Japan, considering also how difficult is to migrate to Japan, the ridiculous limited number of second or third generation Japanese with a European or African descent pool of potential actor. I can also imagine that for foreign talent is not that easy to fly over and work there on any sort of Visa, and probably not economically sustainable.