Any doubts that Fleetwood would take their foot off the gas having guaranteed fourth place appeared to be dispelled in the opening minutes when Steven Schumacher threatened to score before his effort sailed over.
However, the Cod Army did take the lead when Blair latched on to Josh Morris's ball into the box and netted to stun the majority inside the Proact Stadium, who were expecting a title-winning party.
Chesterfield could have edged in front themselves moments earlier when Jimmy Ryan's effort was narrowly wide of the Fleetwood goal.
The Spireites continued to push for an equaliser as Ryan and Eoin Doyle went close to scoring, but were frustrated by the Fylde club.
Indeed, Fleetwood nearly doubled their lead on the stroke of half time when Ryan Cresswell's close-range shot went narrowly wide.
With Chesterfield trailing and Rochdale being held by Newport County, Scunthorpe United are on course to seize the title as they lead York City 2-1.