That brought up Blanks, who blasted a pitch into the second-deck balcony of the Western Metal Supply Co. building, providing the key runs as the Padres reeled off sixth straight victory overall and moved above .500 (35-34).
Will Venable also homered for the Padres.
Clayton Richard (2-5) registered the victory with eight innings of one-run ball. The left-hander didn't issue a walk and struck out one.
Huston Street pitched a scoreless ninth for his 13th save.
Diamondbacks starting pitcher Ian Kennedy, playing while appealing a 10-game suspension for his part in a brawl against the Los Angeles Dodgers last week, drove in Arizona's only run.
The D-backs saw their lead in the National League West shrink to a half game over Colorado.