The goals were Blackstock's first since overcoming a cruciate knee ligament injury that kept him out of action for 13 months and threatened to end his career.
Cotterill was full of praise for the 25-year-old, who has made 11 appearances this season since returning in December.
"There have been signs over the last three weeks that he has been coming back to his old form," he said.
"All credit to him to come back from the injury. It has been amazing. He has had a real bad injury and has shown great character.
"He has done remarkably well to come back and play. That is how bad the injury was and it could easily have finished him."
Blackstock signed from Queens Park Rangers in 2009 and scored 14 goals in his first full season at the City Ground.