"Well, I just -- it's sort of a way of me begging forgiveness," Blunt said before taking the stage. "Chris, I love you. Always have, always will. I'm very sorry."
"And this is a massive sacrifice because I literally loathe singing in front of people," she added.
Blunt stars in A Quiet Place and its sequel, A Quiet Place Part II, both directed and co-written by Krasinski. Martin said earlier this month on Ellen that he sent Blunt and Krasinski an idea for a Quiet Place musical where he was performing with no sound.
"They never replied," Martin said. "I never heard back."
Blunt said Monday that she and Krasinski forgot to reply to Martin, the frontman of Coldplay.
"He sent us this very sweet, funny video of him miming Quiet Place the musical," Blunt told host Ellen DeGeneres. "I watched it 11 o'clock at night next to John, we're about to go bed. I remember going, 'Oh, that's so sweet. That's so funny. I will write back in the morning.' Forgot to write back in the morning."
"A day goes by, then a week, then a month later, I'm like, 'Oh my god! I didn't write back to Chris Martin!'" she said. "I basically felt I left it too late."