HeyUguys claims that the London Boulevard star has been ousted from the project by Warner Bros and Legendary Pictures in favour of an unnamed actor.
Blackley was to portray a character named Callisto, a Greek youth with aspirations to become a heroic soldier like his father. The studios are expected to officially name his replacement soon.
Noam Murro will direct the follow-up to Zack Snyder's original 300. The events of the new film are expected to take place alongside its predecessor.
Sullivan Stapleton, Joel Edgerton, and Eva Green will appear in 300: Battle For Artemisia, which is based on a graphic novel by Frank Miller.
300: Battle For Artemisia is scheduled for release in August, 2013.
> Rodrigo Santoro in talks for '300' follow-up
Watch the trailer for 300 below: