The Luther star, whose role in the Marvel Comics movie sparked anger from a white supremacist group, told The Guardian that he respected Branagh for being "colour blind" in his approach to the blockbuster.
"When Kenneth asked me to be part of it, I did find myself questioning race," Elba said. "But Kenneth hadn't even given that a thought. He just needed an actor who has presence and command, and felt that I fitted the bill.
"It was so refreshing - and a testament to him as an actor and director that his casting was genuinely colour blind. I feel very proud of being part of that movie."
Explaining how Thor fits into his body of work, Elba said: "It's different from anything else I've done; but at the same time, it's been pointed out to me that here I am playing a central, solid figure again - the one that people should trust.
"Take [The Wire's Stringer Bell] or Luther - that seems to be a common denominator in the characters I play."
Thor will open in UK cinemas on April 27 and in the US on May 6.
Watch the trailer for Thor below: