Case in point why can't some Norse god be black. Black people could have wandered that far north and we are talking about a mythical god. Why does anybody give a crap what color he is?
They could have wandered that far north... but they didn't. They did go as far as Rome, as slaves brought back from Africa. The fact is, black/white skin is a function of melanin, which serves to protect one from exposure to the sun. In northern regions of the world, it isn't required as much, exposure to the sun being much more limited. In fact, black people in these regions tend to suffer vitamin D deficiencies in the cold months of winter because of their natural resistance to sunlight absorption.
So while yes, black people could have wandered there, they didn't. The society that believed in these gods was white. Their gods are thus white. Not to mention Heimdall is the friggin White God.
Again, if I made up a fictional story about Nelson Mandela, cast an Asian in the role, but made it so Nelson took control of South Africa in the name of the people by fighting for it in a style of Kung Fu and said "Hey, it's ok to cast Jet Li as Nelson, because this is fiction, the super flying kick of impalement isn't possible, so why are people fretting over this ?"
I don't think me and others are upset over it, I just think it's a weird casting choice and again, Hollywood makes it a tad harder to suspend your disbelief when looking at their mass-market trite. The fact that it is a hate group that is calling for this boycott really doesn't change the fact that the casting is out of place, just as much as casting Elizabeth Taylor as Cleopatra (the most recent Othello movie had Laurence Fishburn in the role and the 1965 version had some white guy that isn't Anthony Hopkins, but yes, Anthony as a Moor of Venice would be the same kind of poor casting choice that would raise this eyebrow).
Not to mention the idealist in me does hope for the day when anyone can play any role.
That's going to be one sad day. It is the day that humans will forfeit their history and culture in the name of political correctness.