Blunt is from Roehampton, London, England. She is one of four children born to a barrister father and teacher mother. Her uncle is Crispin Blunt, Conservative Member of Parliament for Reigate. At the age of 16 Blunt moved to a sixth-form college known for its performing arts programme, Hurtwood House, which is where she attracted an agent.

Blunt lives in London and Vancouver. In the latter she shared a home with her now ex-boyfriend, Canadian singer Michael Bublé, whom she first met in Melbourne backstage at the Australian television Logie Awards in 2005. Bublé thought Blunt was a television producer when they met. She later provided background vocals on his cover of "Me and Mrs. Jones" on his album Call Me Irresponsible.

In July 2008, Blunt ended her three-year relationship with Bublé, due to their busy schedules.