Forest had gone close after just 53 seconds as Paul Anderson found space on the right flank and beat Richard Wood's lunging tackle before unselfishly squaring the ball to Joe Garner.
The midfielder tried to pick his spot drilling his low right-foot strike towards the far corner as Lee Grant pulled off a spectacular stop diving to his right the palm the ball round the post.
The ball was cleared but only to the waiting Anderson, whose delicate cross was met at the far post by former QPR striker Blackstock, who headed in from six yards.
The Sheffield side were stung into action and Luke Varney saw his speculative shot from the edge of the area take a deflection before Marcus Tudgay almost carved out an equaliser with his back to goal, but his dipping shot was tipped over the bar by Lee Camp.
Varney, on loan from Derby, squandered a better opening minutes later after winger Jermaine Johnson had done the hard work producing a precise cross from the left which was poked wide of the target from eight yards by the Wednesday striker.
Forest should have surged further ahead on 17 minutes as Anderson again showed his pace,steaming down the left flank before crossing to the unmarked Blackstock, who fired wide with an open goal in front of him.
Wednesday were back on level terms six minutes before the break. After Varney challenged in the area, the ball fell to Tudgay, who finished with a right-foot volley which took a deflection and wrong-footed Camp for the striker's fourth of the season.
The Sheffield side escaped with Anderson was once again the instigator. Full-back Lewis Buxton was beaten for pace as he cut inside from the left flank to stroke the ball across the face of the Wednesday goal with David McGoldrick failing to connect.