Remember when they were casting for Iron Man’s Black Widow, Emily Blunt was originally offered the role, but she turned it down due to scheduling conflicts with Gulliver’s Travels. At the time, the word was that Marvel would then offer her the female lead in an upcoming project. Well, it looks like the rumour is true, because the studio offered her the role of Peggy Carter, Captain America’s WWII sweetheart.
She turned them down.
The scuttlebutt is that Blunt has her eyes on the role of Catwoman in the third of Christopher Nolan’s Batman movies and won’t take on Captain America to avoid scheduling conflicts. There is also some thought that Marvel does not like to have its people playing in DC’s sandbox and she doesn’t want to jeopardize her chances by pitting the studios against each other. I’m not sure about the accuracy of that rumour considering Ryan Reynolds’ role as Deadpool in Xmen’s Wolverine and the upcoming spinoff hasn’t prevented him from filming Green Lantern (DC).
At any rate, this is still very much conjecture at this time. You can get the complete details over at Batman on Film.