Beginning his on-camera career in the 1950s, Blackwell appeared in TV adaptations of Robin Hood and Treasure Island (as the menacing Israel Hands). Subsequent work included The Old Curiosity Shop (as Isaac List), the biographical thriller 10 Rillington Place (with Richard Attenborough and John Hurt), and the 1991 film Robin Hood: Prince of Thieves, plus episodes of The Avengers, Z Cars, and Dixon of Dock Green.
As a voice actor, Blackwell has worked heavily in radio, animation, and computer games. For BBC Radio, he played Reverend Harold "Stinker" Pinker in two adaptations of Wodehouse's Jeeves books, in 1973 and 1980, as well as the radio version of Tinker Tailor, Soldier, Spy. Blackwell also narrated the Mr. Men animated shorts and storybook tapes, dubbed Vitalstatistix in the live action film Asterix and Obelix Take on Caesar, and voiced Gustav the Magic Mirror in the fantasy mini-series The 10th Kingdom (with Warwick Davis).