Well just look at the beautiful Seoni, she certainly doesn't dark hair. She's Varisian at that, who range from dark to auburn. There are also Aasmars running around, so crossbreeding should account for blonde being about. Not to mention Shoanti shave their hair, so naturally, until they print or say otherwise, a Shoanti could have any hair color.
Certainly not official, but that's about all we have from the stuff they've put out so far. Taking Seoni as an example that not everyone has dark hair there. :)