With Krasinski by her side, she said she "mentioned a friend of mine" and suggested he "should call so and so".
Krasinksi replied: "So and so was so good".
Despite his call to the actress, Blunt said that as soon as she read the script for the film she approached her husband and said: "You need to call her and fire her".
Blunt said she was still friends with the actress "until the movie comes out".
"You didn't send her a script admittedly, she wasn't hired," Blunt defended.
The Mary Poppins remake, entitled Mary Poppins Returns, will take place in 1930s London and sees the magical nanny returning to visit the now grown Michael and Jane Banks and help them "rediscover the joy and wonder missing in their lives".
Blunt will be joined by her Into The Woods co-star Meryl Streep as well as Colin Firth, Hamilton's Lin Manuel Miranda, Emily Mortimer and Ben Whishaw.
Dick Van Dyke will also make a featured appearance.
The Mary Poppins sequel is scheduled to hit theatres in December 2018.