The new deal carries an average annual value of $1.5 million, and keeps St. Louis from potentially losing Blais, 23, in restricted free agency after this season.
Blais, a 2014 sixth-round choice by St. Louis, has six goals and 13 points in 40 games this season for the defending Stanley Cup champions. He was averaging just more than 13 minutes a game last month, before the league suspended the season on March 12 due to the coronavirus pandemic.
He missed 28 games with a wrist injury, which cost him a bit when he returned. When he began the season, he was a second-line forward. After his return, he was mainly on the third and fourth.
Blais was in and out of the lineup last postseason as the Blues marched to their first Cup. All told, he played in 15 of the team's 26 playoff games, posting one goal and three points.
If the regular season resumes, the Blues, with 94 points, will return as the No. 1 seed in the Western Conference, two points ahead of the Colorado Avalanche.