The Blues fell for the first time this preseason after scoring nine times in two previous victories. Chris Mason stopped 25 shots.
Benn sealed the win for the Stars when he stole a puck from former Star Darryl Sydor and beat Mason to give Dallas a 3-1 lead before Tkachuk scored with 6:19 to play.
Sydor, a 17-year veteran, spent parts of 10 seasons with the Stars. He is in camp with the Blues.