The Tangerines survive in the Championship despite the defeat, but it was Harriott - who also scored two in midweek in the victory against Watford that secured the Addicks' place in the division - who stole the show.
The 20-year-old winger struck just after the hour mark for his first, added a second in the final 10 minutes and completed his treble in injury time as Jose Riga's side finished the season with three wins from their last four.
Blackpool were all-but sure of survival coming into the match after taking four points from their last two games, and they started much the brighter.
Charlton goalkeeper Ben Hamer made the first save of the match in the fourth minute, comfortably holding Tony McMahon's long-range strike.
A superb tackle and break from Neal Bishop then presented a chance to Andy Keogh in the sixth minute, but the Irish striker's poor control wasted the opportunity.
The home side threatened again just a minute later as Stephen Dobbie twisted and turned on the edge of the area before dragging his left-footed shot a yard wide of the post.
Blackpool continued to dominate and the superb Dobbie tested Hamer again, jinking his way down the left and forcing the keeper to push his left-footed effort over the bar in the 13th minute.
Craig Cathcart had a great chance to give Blackpool the lead after that but nodded a McMahon free-kick over from six yards.
The home side came close to taking the lead after 35 minutes as Isaiah Osbourne produced a powerful run and crossed for Chris Basham, whose diving header was straight at Hamer.
Charlton had their best effort of the half after 40 minutes as Osbourne brought down Marvin Sordell and Dorian Dervite powerfully struck the free-kick from 35 yards just wide of the post.
Charlton forced Matt Gilks to make his first meaningful save in the first minute of the second half as top-scorer Sordell unleashed a fierce left-footed strike from 18 yards.
Blackpool threatened on the counter attack as terrific work from Dobbie put him through on goal, but he dragged wide of Hamer's left post.
And Charlton silenced the home crowd in the 61st minute by taking the lead through Harriott, who ran on to Sordell's through ball to stroke home from 18 yards.
The hosts almost struck back after 67 minutes as good work from Andy Keogh found Bishop six yards out, but a sliding tackle from Dervite forced the ball over the bar.
Charlton doubled their advantage after 81 minutes as Harriott grabbed his second of the game, stroking home left-footed following brilliant work from substitute Jonathan Obika down the left.
And the winger completed his hat-trick in injury time, latching on to Joe Pigott's pass and striking the ball left-footed low to the left of Gilks.