Purpose of a black Heimdall

I guess I should start with the compulsary disclaimer: This is by no means intended as racism, and not at all a criticism of the actor (whom I think did a good job).

That being said, why did they cast a black actor as Heimdall? Norse mythology has its origin in the Scandinavian countries and originates from around the years 600-700 A.D. At this point, people in Scandinavia had never seen a black person. When they first got acquainted with people of different skin color centuries later, they were somewhat astounded.

Since the Heimdall that the Scandinavian people a thousand years ago most certainly was not black in the people's imagination, why did they decide to make Heimdall black in this movie? And I believe Idris Elba was cast as Heimdall in the original Thor too.