Needing a win to force a decisive seventh game in Chicago, the Bruins looked to have done enough to force the decider when they held a 2-1 lead with under two minutes to play.
But a momentary lapse in concentration saw the Blackhawks score twice in a 17-second span to win the game 3-2 and the series 4-2 to the disbelief of a packed TD Garden.
"It was tough walking in that dressing room and seeing how disappointed everybody was and to try and tell them, as I often say, there's a lot of teams that would have loved to have been in our position tonight," Boston coach Claude Julien told a news conference.
"There's a lot of good things to look at. And what we've been through and what we've accomplished to me is a credit to those guys.
"A loss is hard to take, but you've got to look a little further than that right now."
The Bruins won the Stanley Cup in 2011 but their charge through this season's playoffs had taken on added significance in a city still reeling from the horror off the Boston Marathon bombings and last year's Newtown shootings in neighbouring Connecticut.
Boston had embraced the team like never before, and their run had helped lift the city's spirits a little over two months after two bombs exploded near the finish line of the Boston Marathon, killing three people, including a young Bruins fan, leaving a city dazed and a nation stunned.
"I think that's what hurts the most, in the back of our minds," said Julien.
"Although we needed to focus on our team and doing what was going to be the best thing for our team to win a Stanley Cup, in the back of our minds we wanted to do it for those kind of reasons, the City of Boston, what Newtown has been through, that kind of stuff.
"It hit close to home, and the best way we felt we could try and cheer the area was to win a Stanley Cup. I think that's what's hard right now for the players. We had more reasons than just ourselves to win a Cup."
Boston defenseman Zdeno Chara said he was stunned by his team's defeat.
"It's a tough, tough way to lose a game, tough way to lose a series," he said. "We really felt that we wanted to play as hard as we could, obviously for a number of reasons, and playing for the city was one of them.
"Obviously we tried to have a better result, but it just didn't happen."
Bruins forward David Krejci, who was the leading pointscorer in the playoffs, said his team only had themselves to blame for the loss.
"It feels like we lost it. We had a Game Seven in front of us, It was right there ... we just gave it to them, basically," he said.
"You never want to lose a game like this. You never want to lose a season like this but we did.
"It's not even a point to say that it's going to make us stronger in the future. It sucks that we lost and it's going to hurt for a while."
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