The 27-year-old was offered the role of Captain America's girlfriend Peggy Carter. She passed on the offer and the role went to Hayley Atwell.
Speaking to MTV, Blunt said: "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."
She added: "It just wasn't what I wanted to do next, to be honest."
Blunt also revealed that she was wary of committing to several films for the superhero franchise.
"There's definitely a high commitment level and I'm not someone who likes to plan too much ahead," she continued.
"That would also be an issue for me as well."
The actress will next be seen in sci-fi romance The Adjustment Bureau