Grant Leadbitter had put the visitors ahead with a slightly fortunate goal as his cross found the net off the inside of the post in the 27th minute.
But Dougie Freedman’s side, who tasted defeat for the first time under the new manager at Charlton in midweek, again showed their character as they made it six wins from eight games under the Scot.
Gardner hammered a spectacular 34th-minute equaliser, before Blackstock provided a neat turn and finish to secure all three points with a second goal 20 minutes after the interval.
The defeat saw Boro drop out of the top two, while the victory helped Forest close the gap on the top six to just nine points.
It was only the second time all season that Boro manager Aitor Karanka had named an unchanged starting XI, following their 3-0 win over Millwall in the week.
And he saw his side initially take charge of the game, as Leadbitter’s cross-cum-shot found the back of the net in fortunate fashion.
It bounced back off the far post after hitting Michael Mancienne, although the ball looked to have crossed the line before deflecting off the Forest man.
Forest had previously seen Dimi Konstanopoulos make a fine save to deny Michail Antonio, while Henri Lansbury had twice fired over from the edge of the box. Karl Darlow then had his reactions tested by former Forest man Patrick Bamford.
Gardner had his head in his hands after sending a free header looping over the bar, when Chris Burke had picked him out with a pinpoint cross.
But his frustration did not last for long, as the midfielder bagged his second goal in three games in fine fashion.
Lansbury’s determination allowed him to feed the Aston Villa loanee on the edge of the box and Gardner did the rest, sending a shot across the face of goal and into the top corner.
Kike had an effort deflected wide amid a scrappy start to the second half, while Forest wanted a penalty after Tomas Kalas appeared to have handled while blocking a cross from Antonio.
But Forest did manage to make the breakthrough in the 65th minute, following good work from the always dangerous Antonio.
The winger looked to have set up Ben Osborn for a clear shot on goal, but Blackstock snatched the ball off his toes before turning to deliver a shot high into the corner of the net from 10 yards out.
Lee Tomlin failed to trouble Darlow when a precise cross from Bamford picked him out, with Blackstock also sending another header wide when he could have killed the game off.
It would have been a costly moment at the death had Kike not fired a golden opportunity to level wide of the mark.