Stastny, who spent two years at the University of Denver and played his first eight NHL seasons with the Avalanche, has made a habit of playing well against his former team, with nine points (two goals, seven assists) in 13 games.
WATCH: [All Blues vs. Avalanche highlights]
Stastny tied the game 1-1 at 0:39 of the second period with his third goal of the season and 600th NHL point, and he set up Vince Dunn for a 3-1 Blues lead at 13:25.
"It's cool, it's special," Stastny said of the milestone point. "It's something you don't really think about, just in the moment. When the season is done or my career is done, I'll reflect. My mindset has always been just enjoy the moment. Don't really look at that next milestone, don't get ahead of yourself."
Blues defenseman Robert Bortuzzo scored at 9:05 of the third period when he converted Vladimir Sobotka's pass for a 4-2 lead, beating goalie Jonathan Bernier 10 seconds after the Avalanche killed off Patrik Nemeth's holding penalty.
"It just creeped in," Bortuzzo said. "I don't think (Sobotka) even looked at me until the puck was in the net. It was an incredible pass."
Said Bernier: "That was a tough one. I thought we did a great job on the (penalty kill), kept everything to the outside. I didn't have much to do, just to be in good position."