The Phillies sent infielder Adrian Cardenas, outfielder Matt Spencer and pitcher Josh Outman to Oakland in exchange for Blanton.
Blanton, who was Oakland's Opening Day starter in April, is 5-12 with a 4.96 earned run average in 20 starts (127 innings) this season but was a victim of poor run support.
"The thing about being with the Oakland A's, you tend to be in rumors a lot," Blanton said on the Phillies Web site when told of the trade Thursday night. "I just took the approach that I didn't pay a lot of attention. I was with the team until told otherwise (but) I'm really excited about the situation I'm coming into. They have a great team."
In five major league seasons, Blanton, Oakland's first-round pick in the 2002 amateur player draft, is 47-46 with a 4.25 ERA in 760 2/3 innings.