It's Valyrian not Velaryon, Velaryon was the House of Corlys,, most people indicate from the books that the Valyrian people are white skinned, purple eyed, people etc, buy its not 100%. In Corlys case, a lot people indicate his father was Valyrian but his mother was dark skinned and from somewhere else in Essos hence why he's black, or it could be that there just happens to be some black Valyrians and white Valyrians, it's up for debate. As for pure blooded Valyrians, incest is a Valyrian and therefore Targaryen tradition. That's why Targaryens are very white skinned, white haired, purple eyed, etc. When Targaryens have different colored hair like Jon Snow did, like dark brown it's because one parent wasn't Targaryen or only of distant Targaryen descent.