Nick Blackman had stunned Kevin Blackwell's side with a spectacular goal after the Sheffield side had taken a fifth-minute lead.
United, whose home form has let them down despite creating a new club record of 13 away league games without defeat, had a dream start, breaking the deadlock after Danny Coid headed away under pressure from Greg Halford.
David Cotterill lashed in a low corner from the left flank and the unfortunate Coid steered the ball past keeper Paul Rachubka as he tried to clear.
Blackman, making his first start for the club, struck in the 42nd minute, his low right-foot shot from 20 yards taking a deflection as it looped over Paddy Kenny and into the net after grazing the underside of the bar.
DJ Campbell added a second in the 50th minute, Alex Baptiste threaded the ball to the little striker, centre-back Chris Morgan failed to clear and Campbell nipped in to tuck his low right-foot strike into the far corner from 15 yards.
United were back on level terms in the 72nd minute, substitute Jamie Ward was upended by Rachubka with Cotterill stepping up with a clinical finish as he sent the keeper the wrong way from the penalty spot.
Ward twice forced saves from the Blackpool keeper with stinging shots from outside the area while Kenny pulled off a spectacular stop, tipping Charlie Adam's fierce strike over the top.
Ward almost grabbed the winner deep into time added on ,cutting inside from the right flank, his shot flashing inches wide of the far post with Rachubka beaten before Matt Kilgallon headed wide in a nail-biting finale.