The striker has barely featured for Forest this season having missed over a year of action with knee ligament damage, but he ended the campaign on a high with his seventh and eighth goals.
Garath McCleary, who was crowned the supporters' player of the year prior to kick-off, reached Andy Reid's cross and provided Blackstock with a simple tap-in to open the scoring 20 minutes from time.
The forward then missed a trio of headers in quick succession with Pompey goalkeeper Jamie Ashdown epitomising their fighting spirit with three smart saves. But he was a constant thorn in Portsmouth's side and was rewarded again in the 89th minute with another simple finish after Greg Cunningham's shot came straight to him.
Michael Appleton's side, whose relegation was confirmed with a 2-1 defeat to Derby last week, will now drop into the third tier for the first time since 1983.
Forest came into the game having scored just 19 times in 22 games at the City Ground and their profligacy was evident early on.
There may have been nothing riding on the game's outcome but tempers flared at successive corners between home full-back Chris Gunter and Derby loanee Chris Maguire.
The former went to ground following a tussle with Maguire and appeared to retaliate moments later by elbowing the Pompey striker, although the incident was not picked up by the officials.
Both players had a chance to gain the upper hand in that personal duel with shots on goal shortly afterwards.
Maguire's effort was eventually gathered by goalkeeper Lee Camp at the second attempt while Gunter blazed over with his left foot when Cunningham's fizzed cross found its way to him at the far post.
Early in the second half Gunter was again heavily involved as Forest squandered a great chance to take the initiative.
The Welshman dispossessed Kelvin Etuhu, breaking into the box and squaring for Blackstock in space but the striker snatched his chance and the ball flew over the crossbar.
He did make amends, though, when Forest finally broke the deadlock.
Reid's far-post cross found McCleary and he kept his composure to slide the ball across for Blackstock who could not miss.
He nearly doubled his tally but found Ashdown in inspired form with the stopper denying three of Blackstock's headers as Forest piled on the pressure.
If one moment summed up Portsmouth's season it came in the 85th minute.
Camp made a mess of Maguire's corner, fumbling the ball onto the crossbar and into his own net but the assistant ruled the ball had already gone out of play as it came in.
And, as the final whistle sounded at Southampton, confirming Pompey's bitter rivals' promotion to the Barclays Premier League, Blackstock heaped more misery on the south coast club with his second goal.
Cunningham met Reid's corner and Blackstock showed his predatory instincts, pouncing inside a crowded six-yard box to squeeze the loose ball in.
