Welcome to the coop. As far as I know there was no very recent official studies, so most probably you have seen all the information that was published. The "last word" on the subject has not been spoken yet. Personally I use the term recessive black as a "working term" (is there such an expression in English?), and not a name for any particular gene. From personal experience I can confirm, that the suggestion about "possible multiple genes involved" is true (at least 2 different). The recessive black genes I extracted from black minorcas did not eumelanized the wing triangle on e+ cocks. The genes extracted from black Polish did eumelanized the triangle. I have to take reservation that the number of birds in each category was small, so no more definite conclusions from my side.
You can also see the thread below: