Keith Tkachuk scored an empty-net goal with 30.1 seconds left after Colorado pulled goalie Peter Budaj with 1:03 remaining.
Legace stopped 30 shots to improve to 10-1 in his career against Colorado.
Rucinsky's power-play goal midway through the third period was the winner. He scored his seventh and eighth goals of the season.
Bill Guerin and Rucinsky scored second-period goals for the Blues. Colorado countered with goals from Joe Sakic and Wojtek Wolski.
Guerin, who also assisted on Rucinsky's goal, has five goals in his last five games and 17 on the season.
Sakic's goal inched him closer to tying Paul Coffey for 10th place on the NHL's career scoring list. Sakic (1,530 points) is a point shy of Coffey. Sakic has 17 goals this season.
Budaj had a career-best, four-game winning streak snapped.
The Blues haven't allowed a first-period goal in nine straight games since Andy Murray took over as head coach for Mike Kitchen on Dec. 11.
The teams will meet again Saturday night in St. Louis.