Blanco's 14th RBI this season snapped a 1-1 tie with the bases loaded and one out in the top of the ninth.
The winner was reliever Luke Gregerson (2-3).
Closer Heath Bell pitched a one-hit, scoreless ninth, earning his 38th save.
Pittsburgh's run scored on Ryan Doumit's first-inning fielder's choice groundout.
Five batters had one hit for the Pirates, who have lost 18 of 21.
The losing pitcher was Phil Dumatrait (0-2).
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