I don't see why something based on Norse mythology should involve a black person. This is kind of like when they cast Will Smith in I Am Legend. They take a character, that has been described as white, and then they change it for no good reason. I don't think it is racist to expect that things be done properly. Imagine if they made a movie based on the Drizzt Saga (which would be awesome if they didn't fuck it up), and they cast a white person as Drizzt, who is a black elf. It wouldn't make a damn bit of sense, and would piss off a bunch of people, those that love the story and those that have no idea that there is something called "Forgotten Realms". It would be a white person playing the role of a character with black skin. In the end, it is just skin pigment, but it doesn't need to be politically correct since it is just a damned movie. Now if they didn't cast a black person as a Norse god, and referred to black humans as slaves and niggers, then yeah, that would be pretty fucking racist.
Note: I refused to use the term "The "N" Word" as we would all know what I mean, and it would be pointless. I am not referring to anyone by that word, and I personally find the word offensive.