Blackman began her acting career on the West End stage and had supporting or co-starring roles in several fifties and sixties films (including the Titanic saga A Night to Remember). She reached stardom on TV's The Avengers as Steed's partner Cathy Gale (succeeded by Diana Rigg as Emma Peel). The exposure led to film roles in Ray Harryhausen's Jason and the Argonauts (as Hera) and especially the James Bond film Goldfinger as Pussy Galore, as well as Life at the Top, The Secret of My Success, and Shalako (opposite Bond actor Sean Connery). She remained busy on TV, with guest roles on Columbo, Doctor Who, Minder, Midsomer Murders, and Casualty as well as assorted TV movies (including the Agatha Christie outing The Secret Adversary).
Blackman's periodic later movies included Bridget Jones's Diary and 2012's Cockneys vs. Zombies, her final film. Stage credits include playing the Mother Superior in the original London production of the musical Nunsense.