Established in the comics. Movies are a different medium.
Ever see the made for TV Cinderella movie with Brandy? That was a multi-racial show without having much regard for the ethnicity.
What about Dean Cain as Superman? I don't know if it's because I myself am Asian but I knew straight away that he wasn't white or at least purely white. It still worked and hardly anyone noticed.
I think it's time we need to get over the "established character's race" thing when it comes to movies and enjoy it as a different medium. Some actors might be getting overlooked for something as trivial as the color of their skin.
It's not like these are historical movies where Abe Lincoln was suddenly Asian. It's a frikkin comic book. Granted some characters have their ethnicity as integral to their story, others however do not.
For certain characters the race does not integral to the character, but at the same time, it might make things a bit different.
Let's take an example:
Bruce Wayne is a multi-billionaire orphan. He is the son of Gotham. His family was extremely influential and loved. As a grown man he is the CEO of an international company. This role could easily be anyone of a different race than a white man. However, when I think of a rich, snobby, playboy I think of a white man. I think of a young Hugh Hefner. I guess I find it harder to swallow that a black man or hispanic man would fall into that category. I also don't hear of many black families where the wealth went back a few generations. The backstory and family history of Bruce Wayne was tailored from a white perspective. You would have to change that for it to be believable to the general audience.
Some other examples of characters I'd have a hard time seeing with a race change:
Captain America - if you go with the WWII origin black men would not have been given the special opportunity like a white man. Civil rights was not until some decades later.
Wolverine - He's a Canuck, and let's face it, a bit of a redneck. His hair is also a bit of a trademark to his character.
Characters I could be more open to a race change:
Spiderman - A kid growing up in NY with his Aunt. Nerdy and a photographer. This could be anyone.
Professor Xavier - Years ago, I had the thought of Laurence Fishburne being Professor X after his role in The Matrix. You could change character history and have him fighting for civil rights that would parallel his fights for
mutant rights.
For the character, Blade, could a white man play the role? Is the fact that Blade is black really all that important to his character? I don't know much from the character other than what I have seen in the movies. From the movies all we are told is he grew up on the streets and is a badass vampire killer with an attitude.
My point is, if the background and story of a character are already laid out, it might be more difficult to change a character's race without changing it to more fit the new race. How fans would react to that, is impossible to tell.