Also every pic of a Targaryen I can find shows Blond Hair. Seems that is a fairly strong trait in their line.
Let’s assume that Targaryen silver hair is a recessive trait. And we know that they like to marry brothers/sisters. I think the real reason that they like to intermarry is to keep their bloodline pure. There is a theory that their ability to control/hatch dragons is related to their genes, and it’s probably a recessive gene, too.
There were plenty of Targaryens with dark hair. Rhaenys, Baelor Breakspear, Daeron II, Steffon Baratheon, and even Aegon the Unlikely had a black streak in his hair.