Blunt made her professional debut in Bliss a new play commissioned by the National Theatre, at the 2000 Edinburgh Festival while she was still an A-level student. In 2003, she made her screen debut in the British television drama Boudica, about the life of the ancient British warrior-queen who fought the Romans. She also won considerable praise for her performance as the 16th-century queen Catherine Howard in the two-part British television drama Henry VIII.

Blunt's breakout role was as Tamsin in the dark British movie, My Summer of Love in 2004, which depicted a story of deception and lesbian love in the English countryside. She shared an Evening Standard British Film Award for Most Promising Newcomer with co-star Nathalie Press.

In 2005 she co-starred with Bill Nighy and Miranda Richardson in the British TV drama Gideon's Daughter, an original screenplay written and directed by Stephen Poliakoff, in which she played the troubled only child of New Labour spin doctor Gid