|I don't think what you are seeing fits for dendrite growth. Namely the mechanisms under which it happens, per the article you linked "IPC defines dendritic growth as the growth of conductive metal filaments on a PCB through an electrolytic solution under the influence of a DC voltage bias."
If we look at the ground trace we can see that a lot of it is blackened. Which leads us to think that "ah dendrites!". But that becomes suspect when there needs to be a DC bias. Considering that ground trace should be an the same potential across its surface, dendrites no longer fit.
However, if we google "black copper" we will see that there is a compound called Copper(I)_sulfide that looks like it would be worth investigating.