The LA Times recently reported that Emily was offered the role of the female lead, Peggy Carter, in the Captain America. Her reason for dropping out has been disclosed. But now, Batman-On-Film, is now reporting, from an email they received.
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This dude (who’s pretty damned tuned into the Hollywood grapevine) speculates that one of the reasons Ms. Blunt turned down the role is because she may have her eye set on playing Selina Kyle in BATMAN 3...if Chris Nolan uses the character, of course. "
The site also brought up the fact that Emily Blunt was allegedly up for the role of Rachel Dawes in the Dark Knight, since Kate Holmes dropped out. Eventually the role went to Maggie Gyllenhaal.