From Cinemablend:-
Facts about Emily Blunt:-
(1)Five years younger than KH, but still looks more mature.
(2)Rising star who is affordable AND proven her acting worth (Devil Wears Prada).
(3)Looks more like KH/Rachel Dawes than Rachel McAdams does.
(4)She's a british actress, which Nolan MAY prefer (if you look at BB).
Of the two rumoured actresses to take over from KH, I think Blunt would be more likely.
In terms of McAdam's casting, from Latino Review:-
So IF Rachel IS in TDK, she's going to play a bigger role than people think - NOT a 10 minute cameo.
With Katie Holmes out, The Dark Knight casting rumors are flapping around at a torrid pace. Im afraid were going to have to be responsible for starting this particular one.
Obviously Chris Nolan is actively looking for someone to take over Katie Holmes Rachel Dawes character in the new Batman film. Earlier this week rumor had it that they were considering Rachel McAdams for the part, but according to one of our regular scoopers, they may end up going with Devil Wears Prada star Emily Blunt. Word is that the folks behind the Batman Begins sequel are impressed with her, and may bring Blunt in to read for the role.
Our same anonymous source insists that Katie didnt leave the project, but was dropped. Anything youve heard otherwise is just PR spin to save face. Of course, anyone with half a brain could have figured that out. You dont intentionally ditch a recurring role in Batman to star in a crappy romantic comedy with Queen Latifah. Nobody, not even Katie Holmes, is that stupid.
I like the idea of Emily Blunt as Rachel Dawes a lot better than Rachel McAdams. Dont get me wrong, I prefer McAdams as an actress, but somehow she just doesnt seem like she belongs in the smudged brown world of the caped crusader.
For now though, just pencil this in as a wild rumor. My source is one thats been right before, yet even if this is true it only means that Emily Blunt is being considered for the part of Rachel Dawes. It doesnt mean she has it. Sit back and wait for Nolan to make up his mind.

Facts about Emily Blunt:-
(1)Five years younger than KH, but still looks more mature.
(2)Rising star who is affordable AND proven her acting worth (Devil Wears Prada).
(3)Looks more like KH/Rachel Dawes than Rachel McAdams does.
(4)She's a british actress, which Nolan MAY prefer (if you look at BB).
Of the two rumoured actresses to take over from KH, I think Blunt would be more likely.
In terms of McAdam's casting, from Latino Review:-
This is a script review by Latino Review called KILLING PABLO, but the beginning of the article addresses the Katie Holmes rumors.
Hey everybody! El Mayimbe, here wrapping up Los Mes De Los Reyes.
Before I begin, I want to address something.
I love how the trades say that Katie Holmes dropped out of The Dark Knight. Lets keep it real folks. We all know she was kicked to the curb!
In terms of who is replacing Katie, I feel all the net rumors with Rachel McAdams is just fanboy speculation. Keep in mind folks that Rachel McAdams is the current Hollywood it girl at the top of everyones list. She gets offered everything under the sun.
I asked some of my contacts if there is any truth to the McAdams/TDK rumor and they told me it was just that, a rumor.
So IF Rachel IS in TDK, she's going to play a bigger role than people think - NOT a 10 minute cameo.