Blair evoked immediate memories of his wonderful FA Trophy semi-final winner at Luton in March when getting himself on the edge of an inch-perfect cross from Jason Walker with only nine minutes remaining of a tremendous battle at the One Call Stadium.
That was sufficient to earn his side a 2-1 aggregate victory and a Wembley date with Luton Town later this month. The Hatters eased past Wrexham in their semi-final but next face opposition who have already beaten them three times this season.
It certainly wasn't pretty during the opening half as both sides struggled for fluency on a difficult playing surface. Mansfield were typically relying on the hugely launched throws of Exodus Geoghaghan while their visitors were more measured in approach but nowhere near their best.
As a result the action wasn't great with Patrick McLaughlin trying a couple of efforts for York while Lindon Meikle's low strike never seriously threatened to trouble Michael Ingham as the hosts were clearly missing suspended leading scorer Matt Green.
The Minstermen began the second-half in much improved fashion with Walker and the recalled Lanre Oyebanjo both having early shots charged down and Martin Riley almost turning Jon Challinor's cross past his own keeper. Blair also had a couple of hopeful attempts without sufficient force while team-mates Chris Smith and Chris Doig defended strongly whenever Mansfield broke forward.
Louis Briscoe was though denied by Ingham on the hour but it wasn't until the introduction of big striker Matt Rhead four minutes later that the hosts really began to take a degree of control. Suddenly the Stags were the dominant force with Meikle showing well down the left and he soon drilled into the side-netting while Ingham was then called upon to fend off Riley's header.
But York should have still forced themselves in front on 75 minutes when unattended substitute Adriano Moké blazed well over after good work down the right from Blair.
As the contest moved into extra-time the lively Meikle had a 20-yard attempt turned over by Ingham while Moké struck tamely at Alan Marriott after Ashley Chambers had worked the opening.
When the winner finally came with just nine minutes remaining it sent the travelling fans behind the same goal into total ecstasy as Blair gleefully headed in Walker's superbly delivered cross from the left-hand side of the area. Mansfield had Ritchie Sutton sent-off for a second bookable offence moments later.
Eighteen-goal Walker then spurned two fabulous opportunities to make the game safe during the closing stages when first chipping at keeper Marriott as he advanced before also twice sidefooting straight towards him when left completely unmarked after Moké broke down the left.