Manny Legace hadn't allowed a goal in 127 minutes, 13 seconds and appeared headed for his second consecutive shutout before the Blackhawks scored three times on six shots in a span of 3:23 to erase a 1-0 deficit.
Keith Tkachuk scored for the third straight game for the Blues, who had been 5-0-3 since their last loss on Dec. 13 at Colorado - the franchise's longest unbeaten run in regulation since 2003. St. Louis finished last overall in the NHL last season and had been in the cellar again before going 5-3-3 under new coach Andy Murray.
Goals by Martin Havlat, Petr Bondra and Jeff Hamilton erased the Blues' 1-0 lead, and Martin Lapointe added an empty-net goal. The Blackhawks, who are 4-2 against the Blues, scored three times in the third period in their last trip to St. Louis on Dec. 12, a 3-2 Chicago victory that spoiled Murray's debut with the Blues.
One consolation for the Blues: They haven't allowed a first-period goal in 11 games under Murray, one shy of the NHL record set by the Los Angeles Kings in 1974.
Legace was 5-0-1 with a 1.46 goals-against average in his previous six games, and he stopped the Blackhawks for two periods. His run ended at 6:15 of the third when Radim Vrbata stole Dennis Wideman's clearing effort behind the net and fed it in front to Havlat.
Fifty-nine seconds later, Bondra scored his third goal on the rebound of a backhander by Denis Arkhipov. Hamilton scored his 11th at 9:38 when he skated around the net and beat Legace from the left circle.
LaPointe added an empty-net goal with 44 seconds to go.