Blackman, who starred as Pussy Galore in the James Bond film Goldfinger, plays a pensioner called Agatha in the BBC One show.
Agatha ends up in hospital after being hit by a bicycle and reveals to the hospital staff that she has a past which includes driving ambulances in African warzones. She will also make an impression on paramedic Dixie.
Casualty executive producer Oliver Kent said: "We've had a long history of famous guest appearances but never a Bond girl.
"Honor plays a fantastically witty and charming character on a mission."
The new series also sees the arrival of two new characters - a trainee doctor called Lily Chao, played by Crystal Yu, and a nurse called Rita Freeman, played by Chloe Howman.
The new series of Casualty airs Saturday, August 3 at 9.10pm on BBC One.