The on-loan Blackburn striker finished off two clinical attacks down Motherwell's right wing before converting from close range following Stevie Hammell's first half injury-time free-kick.
Blackman had not scored at Fir Park before Wednesday night but his treble, on the eve of his 22nd birthday, took his tally to eight SPL goals in 10 starts and John Sutton completed the rout with three minutes left.
St Johnstone have now conceded 13 goals in four consecutive defeats and, although they had their fair share of first-half possession, they had no answer to Motherwell's pace in attack.
Manager Derek McInnes had made five changes following the 3-0 defeat to Kilmarnock with Scott Dobie handed a first SPL start and Peter Enckelman, Cleveland Taylor, Steven Anderson and Danny Grainger all returning.
Grainger threatened first when he curled a 25-yard free-kick just wide of the far post but Motherwell, who handed Marc Fitzpatrick a rare start, took the lead in the 12th minute.
Tom Hateley played the ball over Grainger's head for Jamie Murphy to run onto and he drove the ball from the byline into the path of Blackman to tap home from two yards.
Saints tried to fight back and Darren Randolph saved Taylor's deflected effort before the Motherwell goalkeeper held from his own defender after Mark Reynolds diverted Jody Morris's cross goalwards.
But the visitors were leaving far too much space in behind and Motherwell punished them down the right wing again in the 24th minute.
Chris Humphrey played the ball past Grainger and ran clear before calmly cutting the ball back for Blackman to slot into the bottom corner from 10 yards.
St Johnstone did not let their heads go down and pressed Motherwell back for long spells without being able to carve out a clear chance.
Michael Duberry was proving a threat as the visitors forced several corners but could not trouble Randolph.
And Blackman completed his treble in first-half injury-time when he forced the ball home from close range at the second attempt after Stephen Craigan had helped on Hammell's free-kick from near the corner flag.
Blackman remained hungry for more after the break, gently chastising Humphrey after the winger shot just over from a tight angle following an exciting run.
Dobie headed straight at Randolph following Duberry's flick-on and Liam Craig flashed a shot just wide from long range, but St Johnstone were otherwise lacking invention and Peter MacDonald and Collin Samuel came on for Dobie and Sam Parkin in a bid to turn the tide.
Motherwell's grip on the game strengthened though. The impressive Humphrey was again played clear down the right when John Sutton's pass cut the visitors open, but he was felled by a rash challenge by Enckelman outside his box.
The home fans shouted for a red card but referee Crawford Allan was right to show a yellow as Humphrey had played the ball too far to have a sight of goal.
Blackman was given a rest for the final 20 minutes with Saturday's trip to Easter Road in mind. Esteban Casagolda came on and almost set up a fourth with a brilliant low cross that Duberry diverted into the arms of Enckelman.
Randolph saved well from Samuel's close-range header and Motherwell compounded the visitors' misery when Sutton's shot deflected past Enckelman following a penetrating run from substitute Alan Gow.
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