The 31-year-old actress, who was pregnant during the film's shoot, revealed she caught Streep as the veteran star was about to fall after her foot got caught in the cape, reported Us magazine.
"I did save her life. We were rehearsing a scene where she's playing the witch and she's supposed to jump onto the table with a cape and everything. Meryl Streep's foot got caught in her cape and we just started to watch her slowly topple head-first toward the concrete floor.
Blunt said as a payback, Streep, 65, should be lowly dressmaker, where she would play Queen of Versailles.
"She owes me. She should play my lowly dressmaker. She owes me bigtime. She was tormenting me in ('The Devil Wears Prada'), and now she's tormenting me in this movie. She's this witch that has cast this horrible spell on our house. I just said, 'This is it.' We couldn't have any other dynamic now. 'You just have to hate me in films.'"