Snuggerud thought he scored again from in close off a feed from Thomas at 12:19 of the third period, but the Ducks successfully challenged for offside.
Pius Suter scored into an empty net with 4:02 left for the 4-0 final.
The Ducks had three of the four power plays in the scoreless first period, but were outshot 13-7.
They went 0-for-6 with the man-advantage with just two shots on goal.
"Might have had a decent one to start and after that we didn't get any zone time," Ducks coach Joel Quenneville said. "They pressured. They got on us before we got anything moving around and going through. They cut off shots, they blocked a ton of shots tonight in 5-on-5 as well. They did what they had to do to be successful."
NOTES: The Blues have won 11 of their past 12 games against the Ducks and six straight at Honda Center. ... Snuggerud has 14 points (five goals, nine assists) in his past 11 games ... Justin Holl also made his St. Louis debut after the 34-year-old defenseman was acquired from the Detroit Red Wings on Friday for defenseman Justin Faulk. Holl assisted on the empty-net goal. ... Blues defenseman Colton Parayko (back) missed his fifth straight game. ... The Blues have allowed nine goals in their first six games following the Olympic break. ... Thomas stretched his personal point streak to six games (five goals, five assists). ... Dylan Holloway assisted on Snuggerud's goal to give him nine points (four goals, five assists) in six games since returning from injury. ... Hofer is 4-0-0 since the Olympic break, and 4-0-0 against the Ducks in his NHL career. ... Granlund returned after missing six games with an upper-body injury sustained while playing for Team Finland in the bronze medal game at the Olympic Winter Games Milano Cortina 2026. He was minus-2 in 18:44.