Reading have moved quickly for Blackman to beat Crystal Palace to his signature after the Championship club made a bid for the highly-rated forward.
Sheffield United were reluctant to lose Blackman, but they have now accepted Reading's offer for the 23-year-old.
Blackman caught the eye during United's clash with Reading in the FA Cup on Saturday and Royals boss Brian McDermott has moved to bring him in to bolster his attacking options.
Blackman has impressed for Sheffield United this season since his arrival from Blackburn last summer, scoring 14 goals so far.
Blackman is expected to sign a four-and-a-half year deal at the Madejski Stadium once he undergoes a medical.
The striker will become Reading's fourth signing of this transfer window following the captures of Stephen Kelly, Hope Akpan and Daniel Carrico.