Many people assume Jon Snow from Game of Thrones is a Targaryen, but Targaryens have white hair.
Are there any other references mentioned in the book about this?
Alert if there are any spoilers.
The dark hair seed is stronger than the Targaryen silver hair. Remember John Aryn's last words "the seed is strong". That means that the dark hair gene is dominant.
North people tend to have dark hair, and so was Lyanna Stark.
That's why Jon inherited Lyanna's hair.
References from the books (thanks Chanandler Bong).