Gareth Wardlaw headed the hosts in front at St Mirren Park when he glanced home Paul McGowan's cross in the 37th minute and the Paisley side looked set to frustrate their visitors.
However, Blackman claimed his 10th goal of the season and possibly his best when he lashed a strike high into the net with 15 minutes left.
Michael Higdon was then sent off but the home side held on to continue their recent upturn in results.
A tight contest was on the cards from the start, with the first efforts coming from two Motherwell free-kicks.
Alan Gow took the first, a dipping 25-yard strike that was pushed over by Craig Samson, before Tom Hateley's deflected effort from closer in took a major deflection and bounced off the post.
Higdon was booked for an off-the-ball clash with Motherwell goalkeeper Darren Randolph as St Mirren tried to force their way forward.
The visitors were looking slightly sharper in possession but St Mirren were closing them down quickly and Motherwell continued to be restricted to long-range efforts, although Blackman was not far over from 25 yards.
David van Zanten volleyed narrowly wide at the other end. Higdon took the risk of tripping Jamie Murphy on the halfway line after being shown up by the Motherwell youngster's skill but referee Alan Muir, who had harshly booked Saints midfielder Patrick Cregg moments earlier, kept his cards in his pocket.
St Mirren took the lead after a simple cross from the right from Paul McGowan, who was back in the team after missing the 1-0 defeat by Celtic under the terms of his loan agreement.
McGowan had won a corner from nothing and, after his delivery came back to him, centred for Wardlaw to glance a header just inside the far post from six yards.
Higdon was proving a handful and Motherwell centre-back Mark Reynolds, who received his first yellow card in 18 months last weekend, was booked for hauling him down before the former Falkirk striker twice came close from 20 yards in the final stages of the half.
Motherwell had more urgency to their passing after the break but they were struggling to get in behind St Mirren, who got 10 men behind the ball whenever their opponents had possession.
Steve Jennings was booked even though his reckless tackle did not connect with Higdon, who soon flicked on for Wardlaw following a long throw but the goalscorer sliced wide.
Higdon almost made it two when his driven free-kick took a deflection but the grounded Randolph managed to block with his outstretched leg.
Gow finally forced the St Mirren defence to turn when he threaded the ball inside Jure Travner for Chris Humphrey to run on to, but the winger's cutback was placed wide by Jennings from 16 yards.
Saints hit back immediately and Reynolds almost turned Travner's low cross into his own net before Wardlaw was booked for a dangerous two-footed challenge on Tom Hateley.
New Scotland cap Steven Saunders volleyed well over after a Motherwell corner as the visitors struggled for ideas.
But Blackman once more proved his worth to Craig Brown's team when he netted a goal out of nowhere.
The on-loan Blackburn striker collected Stevie Hammell's pass on the edge of the box, turned his man and blasted the ball into the roof of the net.
The forward earned himself a booking for throwing himself into the jubilant Motherwell fans and they were almost celebrating again after Gow played a one-two with substitute John Sutton but the striker curled just wide from 15 yards.
Higdon was sent off in the 80th minute after fouling Stephen Craigan but Motherwell could not exploit their advantage with the closest they came when sub Ross Forbes fired just wide from 25 yards.