The victory takes Blues out of the bottom three ahead of Portsmouth on goal difference.
But these hopes had seemingly disappeared in a frantic game when former Blues midfielder Robbie Savage scored a face-saving equaliser - his first for Blackburn since his move from St Andrew's last season.
Savage's 78th-minute strike cancelled out a 62nd-minute goal from Nicky Butt and it appeared as if Blues' fight to avoid the drop had disappeared.
Forssell then popped up to grab the vital winner with his first goal from open play in two years to keep the relegation battle at boiling point.
There was a certain amount of irony in the result as it was achieved at the expense of a Blackburn side containing former Birmingham favourite, Savage.
Arch-villain Savage has thrived on his provocative approach to games but on this occasion he was most subdued until his equaliser.
The inexhaustible Welshman was often left chasing shadows and his inability to take control of the midfield allowed Blues to enjoy long spells of domination.
Only the brilliance of Brad Friedel prevented Blues recording the early goal they desperately required to put them in the driving seat.
The American goalkeeper was at his best to finger-tip a powerful shot on the turn from Julian Gray over the bar.
Blackburn were able to weather Blues' initial onslaught and the confidence instilled by Friedel appeared to give Rovers added impetus to take the game to the opposition.
Savage's mantle is now being worn by Jermaine Pennant who again produced a performance full of skill and perception which was ultimately cut short before the interval.
Pennant created a good opening from which Emile Heskey agonisingly headed over the bar from close range. Friedel was again alive to a potentially dangerous situation when he finger-tipped a Damien Johnson rocket shot over the bar.
But in the 37th minute there was a significant change made to the game when Pennant was forced to limp out of the action with a leg strain. Surprisingly he was replaced by the previously out-of-favour defender, Mario Melchiot.
Blues piled on the agony for themselves two minutes before the break when Gray produced a short cross into the middle and Chris Sutton completely missed his kick just yards from an open goal.
With Blackburn's defence reeling Melchiot sent Gray clear and the youngster's long-range effort roared into the side-netting.
Blackburn eventually came into the game at the start of the second half with Steven Reid finally producing a worthwhile shot which beat Maik Taylor only to go just wide.
Blues lost their shape as they allowed Blackburn to dominate the early second-half exchanges and only the woodwork saved them when Zurab Khizanishvili's glancing header bounced back off Taylor's right-hand post.
But Blues bounced back to take the lead when Heskey mis-fired to put Gray in possession to cross into the middle where Butt scrambled the ball home in the 62nd minute.
Savage hit home his equaliser after Craig Bellamy's shot had been blocked in the 62nd minute.
Then St Andrew's exploded when Forssell converted Gray's left-wing cross five minutes from time.