James Beattie had grabbed his 13th goal of the season in the 23rd minute, but United failed to kill off the Bloomfield Road side who hit back midway through the second half.
The Seasiders, fresh from a 4-0 trouncing of Coventry City, carved out the first half-chance with a slick three-man move.
Michael Flynn's looping cross found Andy Morrell. He picked out Gary Taylor-Fletcher who volleyed over the top from 15 yards.
The Sheffield side should have snatched the lead in the ninth minute when diminutive striker Billy Sharp got the better of Ian Evatt who pulled him back in the area.
The Blackpool centre-back was promptly red carded by referee Stuart Attwell but Beattie responded with a poor spot-kick that was smothered on the line by Paul Rachubka.
Beattie made amends 14 minutes later with the simplest of headers from six yards after the hard-pressed Blackpool defence had failed to cut out Sharp's cross from the right flank.
Blackpool were almost back on level terms minutes later as striker Ben Burgess' long-range strike rattled the United bar with Paddy Kenny helpless.
Rob Hulse, almost match-fit after recovering from a broken leg sustained at Stamford Bridge in March, came on a minute before the interval in place of Beattie, who had been injured in a collision with the Blackpool keeper.
Chances piled up for United but they squandered the lot, Keith Gillespie slamming his low strike straight at Hulse before Chris Armstong crashed the ball over the target from ten yards after neat work by Sharp.
Armstrong pulled a 20-yard strike wide of the mark before Gillespie volleyed high over the bar.
Midfielder Claus Jorgensen grabbed the equaliser in the 68th minute when his free-kick from the edge of the area took a deflection on its way past Kenny.