Buchnevich, 26, is a restricted free agent with arbitration rights. He played all 301 games in the NHL with the Rangers, who selected him in the third round in 2013. Buchnevich had 79 goals and 116 assists in those games, including 20 goals in 54 games last season.
"He's going to touch all aspects of our game and he's at a good age,'' Blues GM Doug Armstrong said. "As we continue to retool this roster, I think he's a good piece for us.''
Blais, 25, has one year left on his deal at $1.5 million against the cap before he reaches restricted free agency, also with arbitration rights. Primarily a defensive forward, Blais has 35 points in 119 games in four seasons with the Blues. He was a sixth-round pick for St. Louis in 2014.
The trade ends multiple seasons of trade speculation surrounding Buchnevich, despite the Rangers forward being one of their most productive. His 2.67 points per 60 minutes at 5-on-5 were second to Artemi Panarin (3.04) on the team in 2020-21.