The London-born star, who made his name internationally as drug-dealing Stringer Bell in US series The Wire, plays the guardian Heimdall in Thor, which also stars Sir Anthony Hopkins.
He said: "There has been a big debate about it: can a black man play a Nordic character? Hang about, Thor's mythical, right? Thor has a hammer that flies to him when he clicks his fingers. That's OK, but the colour of my skin is wrong?"
The actor also warned against pigeon-holing his title role in the new series Luther as "a black detective".
Idris, an associate producer on BBC crime drama Luther, told the TV Times: "I think we'll put ourselves in a corner if we just describe Luther as a black detective.
"There haven't been many in the past, but the fact that he's black is neither here nor there.
"We all know someone from a different ethnic background, we all mix, so it's great to have a character who happens to be black in the central position.
"He still bleeds, just like anyone else."