Blunt is expecting her second child with husband John Krasinski.
The 33-year-old would-be mother wore a printed sundress as she celebrated with her "The Huntsman: Winter's War" co-star, reported US magazine.
Actress AJ Johnson was also present and shared a picture from the afternoon party.
"An afternoon of love, laughs and friendship celebrating a special piece of my (heart) #EmilyBlunt with hostess with the mostess @charlizeafrica," she captioned it.
Blunt was presented with a cake that was frosted pale-blue and topped with yellow ducks.