Anyone know why there is a black dude playing the "Whitest of Gods"?

So, this might be old, but I just found out about it today. I've seen the Thor trailer a bunch of times, and I just thought the black dude in it was a baddie. I don't ever remember there being a black dude in Asgard. Today, I find out that he's actually Heimdall, the watcher of the borders between Asgard and everywhere else. Not only that, Sif's in the movie too, and for those of you that don't know, Sif and Heimdall are siblings, and there's a white chick playing Sif...

I should have asked this from Chris Hemsworth at C2E2, but I didn't know about it until today.