If you’re looking for some manga with a dark skinned uke that aren’t tribal, here are some
-deliverance
-Ennead
-private scandal
-the titans bride
-a paramour
-dear door
-black sun
-classic BL theatre
-mermaid’s egg
-devotion is my salvation
-Hugo
-kichiku encounte
-tsuki to yabanjin & Erv (sequel)
-tsumi to kuchizuke
I don’t remember them all but there is also a tag for dark skinned
http://www.mangago.zone/tag/Dark+Skinned/
Racism and colorism pretty much is the reason for that but things are getting better. check out this:
https://myreadingmanga.info/hakane-raion-gotoki-no-kuni-kara-eng/
Yes he is from a tribe but it's a real one in which the author respectfully portrayed them in an accurate way plus he dresses modernly he just happens to be from a tribe in Kenya. The best thing is that the author seems self aware of how Japanese people can be prejudice sometimes and discuss race differences in a mostly progressive way, I was pleasantly surprised.
Why is it so hard to find a black seme/uke who is not someone from a tropical forest or a slave? The art style is super pretty though, especially the eyes! Its just the plot that gets me. The only black people I've seen so far are slaves, and the only white people are nobles.