1 - Serum Testosterone Levels in Healthy Young Black and White Men , i only found the abstract, don't have access to the whole study not even with my college info, if you do please share what you find
2Serum Estrogen, But Not Testosterone, Levels Differ between Black and White Men in a Nationally Representative Sample of Americans
Those were the 2 studies i read.
And melanin can influence vit d production which would influence test production, but going on this route it would make sense for darker people to actually have decreased test and not the other way around