Gregory Campbell scored with 7 minutes left in the third period to spoil Halak's hopes for a fourth shutout in five games. That ended his shutout streak at 150 minutes, 3 seconds.
But T.J. Oshie and Brad Boyes both beat Tuukka Rask in the shootout. Halak stopped Patrice Bergeron and Michael Ryder, with help from the post both times. The Bruins also had two goals erased by replays in regulation.
Halak and the Blues have won six straight games – their longest winning streak since 2003. They have just one loss in regulation all season, and their four overtime games is the most in the NHL.
In the shootout, Bergeron put his shot off the left post, and Oshie went under Rask's pads to make it 1-0. Tyler Seguin faked right and went left for a backhand that tied it, then Boyes scored to put the Blues back in the lead. When Ryder's shot appeared to deflect off Halak's glove and then the post, the Blues had clinched the win.
The Bruins have lost back-to-back games for the first time, losing to Washington 5-3 on Friday night after Rask came in to replace starter Tim Thomas in the third period, trailing 3-0. That leaves Rask with all four of Boston's losses this season.
But he matched Halak in regulation, stopping 34 shots in his best performance of the season.
And the Boston offense just missed giving him the win.
Ex-Bruins center Vladimir Sobotka scored on a giveaway from Boston defenseman Dennis Seidenberg late in the first period.
The Bruins saw the red light go on twice, but both goals were erased on replay.
In the first period, Nathan Horton did a spin move from the right circle and sent the puck off the far post. It bounced behind Halak, who reached back and covered it with his glove as the light went on; the referee immediately signaled no goal.
In the second, it was Horton again who lost the goal on review. He made a quality move to get around defenseman Eric Brewer, who hooked Horton down as he put the puck off the top corner of the net on Halak's glove side. It bounced onto the goal line and trickled around but, replay showed, didn't go into the net.
The Bruins finally tied it when Ryder brought the puck over at the left side and centered it to Campbell, who one-timed a shot as he dropped to his knee.
Notes
Andrew Ference and David Backes fought early in the second, and Mark Stuart and B.J. Crombeen mixed it up later in the period. … Bruins F David Krecji had to be helped off the ice after a hard collision with Oshie in the first minute of overtime. … It was the only meeting between these teams this season.