I've yet to see the movie, so I cant comment on the content of the roll, and given how fantastic and ultimately dismissive Thor the comic can be of Norse Mythology, a black Norse god hardly seems like the biggest liberty Thor the movie is going to be taking with the mythological subject mater its using. http://satwcomic.com/nordic-halloween
But at the same time, lets play a little game of hypothetical that may get me into a little bit of trouble.
Lets take the same basis plot of Thor, only switch it up with a different mythology. For the purposes of this example lets use Haitian Voodoo.
So our new comic follows the Loa (Loa are basically the caretakers of the world, something between and angel and a god, they are often analogous to Catholic Saints) Papa Legba, the intermediary between humans and the loa, as he is cast into the human world for some slight by Baron Samedi lord of the cross roads. He becomes something of a hero as he battles against the Voodoo Satan figure Kalfu. There's some romantic tension between him and Erzulie the loa of love, beauty, jewelry and peculiarly gay men. Now say for all this, there was a minor character, an important figure in voodoo mythology, Loko the Loa patron of healing. Say in the film adaptation of this comic a white actor was cast to play the roll.
Would people still complain about his ethnicity? Would it be considered racist of they did? Would it actually be racist?