Blackett sliced a low Franck Moussa cross into his own net past a motionless Joe Lewis in the 63rd minute and his own goal proved to be the eventual winner.
High-flying Posh needed only six minutes to take the lead courtesy of a classy Dean Keates penalty. The midfielder made no mistake after referee Andy Haines pointed to the spot for a foul on George Boyd.
Posh dominated the opening exchanges but were stung by a sensational Southend equaliser which arrived against the run of play as Frenchman Jean-Francois Christophe capped the visitors' first spell of pressure with a glorious overhead kick from 15 yards.
Blackett's error was the defining moment from an often scrappy second period before on-loan striker Theo Robinson was inches away from adding a third Southend goal with a 20-yard lob that finished on the roof of the net.