Coventry appeared to be heading for three much-needed points in this Championship clash after substitute Gary Deegan marked his first appearance of the season following injury with a 78th-minute equaliser, and then Lukas Jutkiewicz popped up with a well-taken strike to send the Ricoh Arena into ecstasy.
But Southern's 91st-minute header from Stephen Crainey's corner ensured the match finished all square in dramatic fashion.
Gary Taylor-Fletcher's stunning first-half strike had initially looked to have piled the misery on the struggling Sky Blues and secured Blackpool's first away win in the league since the opening round of games - when the 30-year-old's carbon copy proved enough to see off Hull 1-0.
Third-bottom Coventry went into the game having made their worst start to a season in 12 years while the Seasiders were looking for their first away victory at the Sky Blues at the seventh attempt since 1937.
Richard Keogh tested Matt Gilks with an early header from Gary McSheffrey's corner before Craig Sutherland saw a well-taken finish ruled out for offside at the other end having delightfully taken down Taylor-Fletcher's incisive through-ball.
Sutherland was played clean through yet again in almost identical circumstances after 13 minutes but this time Joe Murphy saved well at the second attempt.
The Sky Blues were failing to deal with Blackpool's passes over the top and it was from yet another that the visitors broke the deadlock after 18 minutes.
Coventry-born centre-back Ian Evatt floated a deep ball to the opposite side of the penalty area and Taylor-Fletcher took it down well before sending a sweet right-footed half-volley into the far top corner.
Coventry were largely toothless in attack despite plenty of endeavour getting into the final third, but three quick openings towards the latter stages of the first half provided a glimmer of hope of getting back into the game.
Roy O'Donovan was first to head over before Jutkiewicz nodded wide, with the latter then firing straight at Gilks when he really should have done better.
At the start of the second half it was Blackpool who went close to doubling their advantage when Taylor-Fletcher's shot was well saved by Murphy before the offside flag went up.
The hosts continued to struggle in the final third despite all their efforts, with Gilks still yet to be tested in the second half when Sammy Clingan's low 67th-minute free-kick flew narrowly wide.
City boss Andy Thorn turned to no-nonsense midfielder Deegan and striker Clive Platt in a bid to turn things around and it worked as Clingan's corner found Deegan 16 yards out to level matters up.
Five minutes later and Coventry were ahead as Jutkiewicz created space in the penalty area before drilling low across Gilks.
But the home side could not hold out and one minute into the six added on, Southern headed home Crainey's corner to level things up.