Blum drove home the winning run in the Astros' last at-bat for the third time this season. He did it in consecutive games against the Chicago Cubs in June. Houston won for the ninth time in their last at-bat.
The third baseman was a hot topic on local sports radio Friday after yelling at a fan who booed him during Thursday's 9-4 victory over Washington. Blum led off an inning with a double and said the fan called him out for not making it a triple.
"After last night, it was interesting to say the least," Blum said. "I got a chance at redemption and I took advantage of it."
Michael Bourn drove in three runs and assisted on two putouts at the plate to help the Astros (43-43) reach .500 for the first time since they were 1-1 on April 7.
Houston manager Cecil Cooper said Bourn is the Astros' most valuable player in the first half of the season. The speedy center fielder went 2 for 5 and stole a base.
"It's always a compliment to hear your manager say that," Bourn said. "That's a good compliment for the first half, but I've still got a long way to go."
Oswalt allowed three runs and five hits before leaving in the seventh with numbness in two fingers in his right hand. He hurt his hand when he broke his bat on a groundout in the sixth, but he expects to make his next scheduled start, the first game after the All-Star break.
"I'm sure I'll be fine," he said.
Tejada led off the ninth with a single against Joe Beimel (0-5), who retired Lance Berkman before being replaced by Mike MacDougal. Tejada moved up on Carlos Lee's fly out to center and Hunter Pence was intentionally walked.
MacDougal struggled with his control after putting on Pence, throwing a wild pitch and walking Ivan Rodriguez before Blum singled to right to end the game.
The Nationals have lost five straight and nine of their last 11.
"It's at the point where it's just embarrassing for us," Beimel said. "I think everyone in the clubhouse should be embarrassed about the way we play and just the way we lose games.
"We find a way all the time. It's just sickening."
Beimel gave up Jeff Keppinger's tying sacrifice fly in the eighth. Blum helped set up the run when he doubled after the Nationals misplayed his foul pop earlier in the at-bat.
Jeff Fulchino (3-3) got the win with some help from Bourn in the ninth. Nyjer Morgan hit a one-out single to center with Willie Harris on second but Bourn made a perfect throw to the plate to keep the game tied at 5.
"I was trying to get rid of it pretty quick," Bourn said. "I just knew if I threw it on line, I'd have a good chance."
Lee and Bourn hit leadoff homers in the second and third off Scott Olsen to put Houston up 2-0.
Oswalt retired the first nine batters. Morgan led off the fourth with a single to right, then stole second and third. He scored on Nick Johnson's one-out double to left-center to cut the Astros' lead to 2-1.
Johnson tried to score on Adam Dunn's single to center, but Bourn threw him out at home.
Bourn drove in two runs with a single in the fourth. Oswalt retired seven of eight hitters before running into trouble in the seventh.
Dunn and Josh Willingham led off with consecutive doubles and pitching coach Dewey Robinson came out to check on Oswalt. A trainer and Cooper jogged to the mound and Oswalt left after a brief discussion.
- Nationals 3B Ryan Zimmerman missed the game to fly home to North Carolina, where his grandmother died. Zimmerman was expected to rejoin the team Saturday.
- Cooper and RHP Russ Ortiz met in the manager's office before the game. Ortiz was upset when Cooper pulled him after he gave up four runs in three innings on Thursday night. "I explained to him why I took him out, that was all," Cooper said. "We're good to go. He was just frustrated."