If it becomes official, Blunt will make up the third part of some already fantastic casting -- you haven't forgotten that Mickey Rourke and Sam Rockwell are tipped to play the villains, and the Black Widow's co-conspirators. The early details have Natasha being employed as Tony Stark's personal assistant, which would put her right where she needs to be to help out Rourke's Crimson Dynamo. Naturally, there will probably be a love triangle between Natasha and Pepper, if Gwyneth Paltrow does return for the sequel.
It's a fantastic casting choice, and more evidence of how superhero flicks continue to raise the bar in the casting department. There's few who would please me as the Black Widow, and Blunt is definitely one who would. She's gorgeous without being an empty sexpot, and would be entirely believable as a smart, Soviet (ex-Soviet, I suppose) superspy. She's proved her acting chops in numerous films, and this is just the kind of popcorn flick she should be in. Plus, she looks fantastic as a redhead. Pick her, Marvel! Pick her!