Black Goku is technically a past Goku. Zamasu goes back to Goku and Goten working the field in the BoG arc. And when Zamasu went back he made a 6th timeline in the series. And as we know, different timelines and characters in them have no effect on other timeline versions of themselves.
So technically using your theory, Black Goku getting stronger would make present (main timeline) Goku stronger.
I Thought this was Goku after he fought Zamasu for the first time