The deal was worth $22.5 million total, a source told ESPN.com's Pierre LeBrun.
"As much as you'd like to say you're mentally strong and you can block it out, it's something that does weigh on you," Backes said Friday. "I think it's human nature that you're thinking about it now and then.
"The Blues are making a big commitment to keep me here," he said.
The 26-year-old Backes is in his fifth season, all with the Blues, and has two goals, four assists and 32 penalty minutes in 14 games.
Last season the 6-foot-3, 225-pound forward had career highs with 31 assists and 266 hits. He tied for second on the team in assists and with 48 points.
General manager Doug Armstrong said the parties tried to get the deal completed before the season started.
"I was very comfortable that if the deal didn't get done, knowing David's character and his professionalism, he was going to be able to deal with this and continue to play at a high level," Armstrong said. "I know now moving forward, we don't have to worry about what we're going to do.
"He's a Blue for the next five years and I'm hoping much past that to end his career here," he said.
Backes, the Blues' second-round pick in the 2003 draft, likes the franchise's chances for becoming a contender.
"Maybe I don't know any better, but I know this group's trending in the right direction," Backes said. "With a few injuries, we've still been able to maintain a high level of play.
"We plan on doing that, not for just the rest of this year and into the playoffs but for years to come," he said.
Information from ESPN.com's Pierre LeBrun and The Associated Press was used in this report.