The Phillies won their fourth in a row and remained seven games in front of Florida in the National League East.
Blanton (10-7) won for the first time since Aug. 30. He allowed five hits, walked four and struck out seven.
Philadelphia had a 2-0 lead going into the seventh and reliever Tyler Clippard retired the first two men in the inning. Clippard then hit Chase Utley with a pitch and was relieved by Sean Burnett.
After Utley stole second, Ryan Howard was walked intentionally and Raul Ibanez worked a nine-pitch walk to load the bases.
Jason Bermann was called on to face Werth, who hit a 1-2 pitch out to left-center for his 34th homer of the season.
Washington starter Livan Hernandez (8-11) took the loss, giving up two runs on seven hits in four innings.