Crystal Palace 1-1 Forest
And they were left with a mountain to climb when Adlene Guedioura was sent off with 27 minutes remaining at Selhurst Park.
The Algerian midfielder is facing a three-match ban after being shown a straight red card after appearing to kick out at Palace skipper Mile Jedinak.
It looked like being a night of intense frustration for Forest’s superb travelling support before Blackstock popped up to cancel out Dikgacoi’s opener.
Reds boss Sean O’Driscoll made two personnel changes to his starting line-up but altered his approach by starting with the 3-5-2 they had deployed in the closing stages of Saturday’s 2-2 draw with Birmingham.
Simon Cox and Dan Harding returned to the starting line-up at the expense of Blackstock and Guedioura respectively.
Cox earned his recall after launching Forest’s fight-back against Birmingham with a wonder goal after joining the action as a substitute.
Sharp was preferred to him on that occasion - after Cox had picked up a midweek foot injury while on international duty with the Republic of Ireland - but O’Driscoll decided the time was right to unleash them as a partnership from the start for the first time.
It was, however, Palace, buoyed by their Friday night win over arch rivals Charlton, who made the brighter start.
The woodwork came to Forest’s rescue after nine minutes; Wilfried Zaha was allowed room to cross from the right and Glenn Murray headed against Lee Camp’s left-hand upright.
Camp was forced to make a sprawling save to deny Jonathan Williams after Sam Hutchinson had been robbed of possession, while Dikgacoi – Palace’s match-winner against Charlton – fired inches over the Forest bar from outside the area.
The Reds’ No.1 was called into action twice more before the break, to deny Jedinak, who tried his luck from distance, and Murray after Daniel Ayala had squandered possession on the edge of his own area.
Forest were forced to make a change five minutes before half-time as Guedioura replaced Hutchinson, who had picked up a knock after finding himself on the receiving end of a crunching challenge earlier in the game.
The second half was only five minutes old when Palace made the breakthrough, courtesy of Dikgacoi who beat Camp with a looping header from a Williams’ corner.
Lewis McGugan attempted to respond to the setback almost immediately but saw his shot charged down from the edge of the area after he had created space for himself.
Guedioura was booked in the 55th minute and got his marching orders eight minutes later as he aimed a kick at Jedinak right in front of the referee after a soft free-kick had been awarded against Reid.
Forest had a mountain to climb and O’Driscoll introduced Blackstock to the action, at the expense of Sharp, with 17 minutes remaining, and Radi Majewski for McGugan with nine minutes left on the clock.
And just as his substitutions had worked against Birmingham, The Reds boss again worked his magic as Blackstock bagged an 82nd minute leveller.
Cox turned superbly on the edge of the Palace area and when he poked the ball through for Blackstock, the former Southampton and QPR man kept his cool to beat Speroni.
The visitors came close to snatching a last-gasp winner as Speroni made a stunning save to keep out a Blackstock header from Greg Halford’s long throw into the six-yard box.
And Camp matched his Palace counterpart’s heroics as he produced an equally marvellous stop to keep out a Peter Ramage header from point-blank range in a grandstand finish.
Scorer: Blackstock 82'
Bookings: Halford 29', Guedioura 55', Blackstock 75'