Emily Blunt has turned down the role of playing the lead female role in Joe Johnston’s The First Avenger: Captain America. Hero Complex reports that Marvel favored the award-winning actress (as they had previously when they wanted to cast her as Black Widow in Iron Man 2; the role went to Scarlett Johansson when Fox exercised a clause in Blunt’s contract for Gulliver’s Travels that kept her on their movie) to play Captain America’s (Chris Evans) girlfriend during World War II, Peggy Carter (in the present day, her niece Sharon Carter is an Agent of SHIELD).
According to Hero Complex, Blunt has other offers and that’s no surprise since she’s a talented actress and would add some prestige to any picture. There’s also the problem that the female lead in superhero movies is often a thankless role. Additionally, it’s possible that Marvel wanted to tie her to future films as they’ve done with other actors starring in their movies.
Marvel is keeping the search alive with other British actresses such as Keira Knightley and Alice Eve under consideration, it appears the studio is looking to the other side of the pond to find their Ms. Carter.