The hitmakers made a surprise appearance on a recent taping of "The Ellen DeGeneres Show" to help the Brit conquer her fear of singing in front of an audience, and sang their hit song song "I Want It That Way" with Emily.
Fans have always known the actress has a great voice - she performed in "Into the Woods" and will soon be seen as singing nanny Mary Poppins in "Mary Poppins Returns" - but she really wasn't comfortable when she noticed Ellen DeGeneres had set up a microphone and stand on her stage during their TV chat.
As Ellen persuaded her guest to sing something, Blunt said, "I think it's still not a natural thing for me. I find it very nerve-wracking to sing in front of people."
The host then assured her that her studio audience wanted Emily to sing, prompting the guest to reluctantly take the mic.
DeGeneres then offered her the perfect backing band and the Backstreet Boys appeared from behind a partition, prompting thrilled Blunt to exclaim, "Now we can do it!".
The sextet sang in perfect harmony and Blunt really impressed as she was left to sing the last line alone, before pointing at singer Nick Carter and screaming, "You left me alone!".