A combination of desperate defending and wonderful Brad Friedel goalkeeping had kept Birmingham out after Andy Todd's stupid 27th minute dismissal.
Rovers winger Damien Duff had given his side a deserved lead in the 19th minute after a bright start by his start, finishing off Dwight Yorke's cross after Andy Cole had miskicked.
However, Birmingham's talented French World Cup winner Christophe Dugarry was causing problems in more ways than one to the Blackburn defence.
After exasperating both the Rovers players and fans, Todd exacted his revenge on Dugarry by kicking out at him while he lay on the floor.
Todd was shown the red card by referee Clive Wilkes, sparking a final 15 minutes of madness to conclude the first half which saw two more Rovers players booked and the home team survive an almighty goalmouth scramble.
The second half was even more frantic, David Thompson should have doubled the lead but blasted into the crowd after Duff found him skillfully.
Dugarry thought he had equalised after 55 minutes when he met Jamie Clapham's cross with a header but it dropped narrowly wide.
Birmingham boss Steve Bruce sent two strikers in Jovan Kirovski and John on to the pitch, both for defenders in an attempt to get an equaliser.
The move almost paid immediate dividends when Dugarry picked out Kirovski, but a wonderful one-handed save from Friedel denied the Blues.
However, the visitors were to get an equaliser and it arrived six minutes from time. Stan Lazaridis delivered a pinpoint cross from the left and John rose highest to head home past Friedel's despairing dive.
Blackburn could have won it in the dying seconds, Duff flashing a drive inches wide and then the same player being denied by the impressive Ian Bennett when clean through.