Capitals goalie Michal Neuvirth stopped 25 shots to set a franchise record for wins by a rookie with 21. The 22-year-old topped Bob Mason, who won 20 in 1986-87.
Playing in his second game since being acquired in a trade with New Jersey, Arnott took a crossing pass from Alexander Semin and beat goalie Ty Conklin with a slap shot from the left circle. The 36-year-old Arnott got a key assist against the New York Islanders in his Washington debut Tuesday night.
Alex Ovechkin had two assists for the Capitals, who have won two in a row at home for the first time since Nov. 26-28. They're also riding their first three-game winning streak since Dec. 26-Jan. 1.
Alexander Steen and B.J. Crombeen scored for the Blues, who were coming off two straight shutout losses to Calgary. St. Louis has dropped six of seven.
Washington trailed 1-0 until Scott Hannan scored at 3:05 of the second period on a slap shot from the right circle. It was the defenseman's first goal since Nov. 23, 2009 — a span of 118 games.
The tie lasted just over 2 minutes. Crombeen gained possession of the puck off a faceoff in the Washington end, skated up the right side and whipped a shot past Neuvirth for a 2-1 lead.
It didn't take long for Washington to pull even again. Mike Knuble's pass from the left circle was deflected into the air to Niklas Backstrom, who took a hack at it around his waist with a baseball swing. The puck one-hopped into the net past the stunned Conklin at 6:48.
Despite outshooting the Blues 10-6, Washington was held scoreless in the first period for a sixth consecutive game.
Steen put St. Louis up 1-0 at 11:25 of the period, scoring off a fine cross-ice pass from Chris Stewart. The goal ended the Blues' scoreless run at 133 minutes, 32 seconds — dating to the third period of a Feb. 25 win over Edmonton.
NOTES: Washington has an NHL-low 38 first-period goals. ... Crombeen's goal ended his 13-game run without a point. ... The Capitals recalled G Braden Holtby from Hershey of the AHL and assigned G Todd Ford to Hershey. ... The Capitals swept the season series 2-0. The Blues' last win in Washington was on Jan. 28, 2003. ... Steen's goal was only his third in 21 games. ... Ovechkin has at least one point in seven of his last eight games.