Ferguson had no idea actor John Krasinski was even dating his friend when the two men found themselves sitting next to each other on a plane flight from London to Los Angeles.
And he was stunned when The Office star confided in him that he was planning to ask Blunt to marry him upon his return to California.
Appearing on Ferguson's late-night show in America on Friday (04Dec09), Blunt insisted Ferguson was the one who helped "lock" her happiest moment "down".
She told the host, "You guys talked for, like, 10 hours straight."
Blunt, who previously dated singer Michael Buble, is now engaged to marry Krasinski.