The British actress is the latest star to join the cast of the movie musical, portraying The Baker's Wife opposite Streep as The Witch.
Blunt is already looking forward to reuniting with the screen icon, who played her nasty boss in the 2006 comedy, telling Vulture.com, "I can't believe I'm playing in two movies with Meryl where she's playing someone who's horrible to me. I'm so excited."
However, Blunt admits critics have voiced concerns about her singing voice, after a video of her performing karaoke on a night out ended up on YouTube.com.
She adds, "It's horrible! People are like, 'Well, if you want to hear her sing, this is how she sounds.' And I'm just going, 'Yikes!' I hope people will not give me a hard time about it when the film comes out. I'm not the greatest singer in the room by any means, but I'll have fun trying."
Into The Woods, which begins filming in London this autumn (13), will also feature Johnny Depp as The Wolf and British comedian James Corden as The Baker.