Emily Blunt was the first choice to play Black Widow in Iron Man 2, and she was also up for the role of Peggy Carter in Captain America: The First Avenger. But she wound up not playing either role, and she doesn't seem too broken up to have missed out on them.
Promoting her new movie, Salmon Fishing in the Yemen, Blunt told Vulture:
Usually the female parts in a superhero film feel thankless: She's the pill girlfriend while the guys are whizzing around saving the world. I didn't do the other ones because the part wasn't very good or the timing wasn't right, but I'm open to any kind of genre if the part is great and fun and different and a challenge in some way. I would love to do a comic-book movie or a science-fiction film that would scare the bejesus out of me. Maybe I need to be James Bond! I just did Looper, because it's so original and breathtakingly cool. The time-travel aspect is just a backdrop to visit this heightened world, where you're atoning for something and attempting to be more than you've been.
I'm pretty sure the reason she didn't play Black Widow in Iron Man 2 had more to do with timing than the part not being good — although Black Widow isn't exactly a fleshed-out character in that film. Back in 2009, she told reporters she was really disappointed to lose out on the Black Widow role, although she could have just been saying what people wanted to hear. In any case, if she had played Black Widow in Iron Man 2, then she would have gone on to play the role in The Avengers — where, one suspects, Natasha will get a bit more fleshed out under Joss Whedon's guidance. If you judge Black Widow by her appearance in the first film, then yeah — not really a great character. [Vulture]