Boyes' second goal broke a 2-2 tie late in the second period. Ryan Johnson lead Boyes with a short pass to the blue line where Boyes collected it, took two strides into the Washington zone before blasting a slap shot past Brent Johnson with 1:19 remaining.
Christian Backman scored his first goal of the season 1:11 into the third period to give the Blues a 4-2 lead.
The Blues, who suffered their first shutout of the season in a loss at Columbus on Thursday, took little time to end their scoreless streak. Boyes got his second goal of the season 45 seconds into the game on Keith Tkachuk's sixth assist of the season. Tkachuk grabbed a loose puck in the neutral zone and pushed it ahead to Boyes at the blue line. Boyes skated toward the Washington net before putting a shot over the left shoulder of Johnson, making his first return to St. Louis since being traded from the Blues to Phoenix on March 4, 2005.
The Capitals have now lost six of their past seven games after winning their first three contests of the season. They scored three goals in one of their losses and two or fewer in the others.
Washington tied the score with consecutive goals from Mike Green after falling behind 2-0. He got his first of the night, and his second of the season, about midway through the first period and just 40 seconds after Eric Brewer scored to cut the Blues' lead to 2-1.
Alexander Ovechkin earned the assist on Green's score, giving him at least one in each of the past four games. He got his second assist with 1:12 left on Michael Nylander's goal to cut the Blues' lead to 4-3.
Green tied the score with a shot from the left circle that Blues goalie Manny Legace, who was screened by Donald Brashear, got a glove on but couldn't stop.
Notes: Saturday's game marked the only game of the season between the two teams and the first time they met in St. Louis since Oct. 18, 2003. ...Washington has scored more than three goals just once this season and has had two or fewer in all but three of its 10 games. ...Blues R Dan Hinote skated in his 400th NHL game.