The performance of Tennessee Williams' "A Streetcar Named Desire" was canceled but the 40-year-old Australian actress will return to the stage for Thursday's show, Sydney Theater Company spokesman Tim McKeough said.
"Another actor lifted the prop above his head and she somehow sustained a minor blow to the head," McKeough said.
An audience member told Sydney's Macquarie Radio it soon became clear the accident was not part of the play.
"She had blood streaming down the back of her head and blood on the back of her neck," the unnamed theater-goer said.
Sydney Theater Company is run by Blanchett and her playwright husband, Andrew Upton.
Blanchett, who won a supporting actress Oscar in 2005 for "The Aviator," is playing the lead role of Blanche DuBois.