Geekie tied the game 1-1 at 12:17, following his own shot from the slot after it slid through Mrazek’s pads and tapping it in at the top of the crease.
Marchand put the Bruins ahead 2-1 at 2:58 of the second period with a one-timer from the right circle off a cross-ice pass from Pastrnak.
Marchand scored again to make it 3-1 at 5:03, lifting a backhand from below the right circle past Mrazek after a shot by Pastrnak.
Jason Dickinson brought the Blackhawks within 3-2 with a turnaround shot from the left circle at 18:23. The puck deflected into the net off Bruins defensemen Brandon Carlo and Nikita Zadorov.
Geekie’s breakaway goal at 5:02 of the third period put the Bruins up 4-2. Brazeau sprung Geekie, who skated in and scored on a wrist shot.
“It was good,” Geekie said. “He (Brazeau) made a nice play. He found me twice today. You see him make plays like that all the time, so it's no surprise to anyone. I just try to get to the right spot for him and he made a great play.
"I was trying to kind of hide. ‘March’ made another good change and I was trying to hide behind the second D. I honestly didn't say much. I saw him get the puck and tried to take off. Normally that's your first look when you're kind of outnumbered in the zone."