Houston erased a 4-1 deficit by scoring seven times in the top of the fifth inning.
Blum hit a solo shot in the second inning and launched a three-run homer in the fifth.
Miguel Tejada had two hits and three RBI, with one run scored. Brad Ausmus had four hits, two RBI and scored twice. Kaz Matsui knocked in two runs for the Astros, who had lost five of their previous six games.
The winner was reliever Chris Sampson (5-3).
Milwaukee's Ryan Braun had three hits, one a two-run, third-inning homer.
J.J. Hardy had two RBI while Prince Fielder and Mike Rivera had one RBI each for the Brewers, who enter a key four-game series against the Chicago Cubs Monday night with a one-game deficit in the National League Central Division.
The losing pitcher was Jeff Suppan (5-7).