The actress was offered the chance to play Captain America's love interest Peggy Carter in the forthcoming comic book adaptation, but rejected the role because she wants to remain open to other projects.
She tells MTV.com, "I'm a big supporter of (director) Joe Johnston and I think that Captain America is going to be really fun and I gather that the story is really interesting. It just wasn't what I wanted to do next, to be honest.
"There's definitely a high commitment level and I'm not someone who likes to plan too much ahead. That would also be an issue for me as well."
Blunt previously gave up the part of the Black Widow in Iron Man 2, which was later handed to Scarlett Johansson, due to scheduling conflicts, but the 27 year old remains optimistic she'll get to star in a comic book film soon.
She adds, "I don't tend to be fatalistic about these things. I think for me the job always has to be the right thing at the right time. Neither of them happened to be that."
Blunt's fiance John Krasinski was the front runner to play Captain America but the role eventually went to Fantastic Four actor Chris Evans, while rising British star Hayley Atwell was handed the leading lady role.