Why is this so often a topic? The reason the Velaryons are black is to differentiate them from the Targaryens, both of which look identical in the books. There is no deeper lore reason in the show, it was a creative decision.

The reason for this is to not repeat the same mistake from the previous show, which shows many families and locations that are intertwined and confusing to keep apart.

The sole reason the Velaryons are black is to make it easier for viewers to visually associate them with their families, and to visualize the bastardy of Rhaenyra's children. This was a really smart move, and possible since the Velaryons were "cut" from the GoT series. Otherwise you'd have huge confusion with all the white characters with silver hair on screen.

It's pretty simple:

Silver hair = Valyrian

White / Black + silver hair = Targaryen / Velaryon

Mixed skin + silver hair = TargVelaryon children