The former QPR striker opened the scoring from the penalty spot and netted the winner 10 minutes from time after Luke Varney had levelled for the visitors.
Wednesday had the first chance of the afternoon when Varney cleverly exchanged passes with Marcus Tudgay, but Forest goalkeeper Lee Camp got comfortably behind his side-footed effort.
After an uneventful first quarter, an extended spell of possession resulted in Forest winning a penalty in the 23rd minute when Blackstock's shot struck Darren Purse's foolishly raised arm.
Blackstock stepped up and smashed the spot-kick into the top corner, rendering Lee Grant a mere spectator as he dived in the opposite direction.
Moments later Wednesday's Darren Potter was carelessly dispossessed and Grant had to be alert to catch a venomous effort from Nathan Tyson.
At the other end Wes Morgan stopped Varney getting a shot away inside the penalty area following another impressive move with Tudgay after half an hour
In the 32nd minute Grant did well to punch a curling Earnshaw free-kick to safety after Chris Cohen was brought down on the edge of the penalty area by Michael Gray.
It was Gray who fashioned Wednesday's next opportunity when Tommy Spurr headed his corner wide in first-half stoppage time.
The visitors were almost level in fortuitous fashion at the start of the second half when Darren Potter's free-kick crashed into the crossbar after a deflection left Camp helpless.
And with 18 minutes remaining Wednesday had an equaliser befitting of the scrappy contest.
The home side could only clear Frank Simek's long throw as far as substitute Etienne Esajas and his raking cross finally broke to Varney who applied the finish.
Wednesday found themselves in the ascendancy and it came as a surprise when Forest regained the lead in the 80th minute.
Blackstock got in front of the Wednesday defence to head Chris Cohen's dangerous free-kick towards goal.
Grant parried but Blackstock reacted quickest in a crowded penalty area to lash home his second of the game from close range.
Forest were made to sweat when the fourth official signalled an unexpected five minutes of stoppage time but they were able to withstand a late Wednesday onslaught.