You know how like all of the images of the ancestors and how they all have pitch black skin? Does this mean that the older trolls get the darker their skin gets...?

No, the ancestors are drawn so that they have very little details shown.  Kinda like how the kids’ guardians are all blank white with very little detail.

But if you’d like to headcanon that trolls’ skin gets darker with age that’s totally cool and up to you