I'm confused why you think "blond or red-headed" has anything to do with whether you have any viking ancestry. Just because my father is blonde does not mean I am. And the point of the article is that our lines merge much sooner than we intuitively believe.
If you are black in any society that has had a mix of European and African ancestry more than ~200 years ago, you are almost certainly partially of European descent. Unless somehow all of your 200+ descendents avoided any sexual contact with any colonial. If you're of Western European descent, you are almost certainly of Viking (i.e. Danish/Norse) descent: they spent centuries pillaging and colonizing different areas of Europe (including England). Therefore, if you're black in the Americas, you're almost certainly descended from both Vikings and the ancestors of the Zulu (considering the Zulu were a kingdom from the 19th century, so you're right, it's unlikely everyone with European ancestors, or even everyone with African ancestors, is). If you're from any country that was a European colony more than 300 years ago, this is likely true, too.
And remember, cross-continental trade is old. People from the African continent have been having children with people from the European continent for thousands of years.