"I thought it was a home run off the bat," Weiss said.
Blackmon jumped just at the wall and caught the ball near the top of the fence with one out in the ninth inning Thursday, preserving the Rockies' 4-3 win over the San Diego Padres.
Nolan Arenado hit his NL-leading 38th home run for the Rockies. Carlos Gonzalez hit his 37th homer, and Daniel Descalso hit a tiebreaking shot leading off the eighth. Corey Dickerson also connected for Colorado.
The game was a make-up of a rainout in July 19, the second rainout at Petco Park, which opened in 2004. It was also the hottest gametime temperature in Petco Park at 93 degrees.
"That part was tough, knowing that we were in Southern California, but it felt like Atlanta," Arenado said. "We've played in weather like this before. To win like that was kind of weird, but we will take it."
Descalso homered off Joaquin Benoit (6-5), who had not pitched since Saturday because of back spasms.
"I was just trying to get on base there looking for a good pitch to hit," Descalso said. "He left something out over the plate and I was able to put a good swing on it and drive it. I hit it really well, but this a big yard. But I was pretty sure it was gone."
Arenado has homered in eight of his 11 games in September. He leads the league with 108 RBIs.
"I'm just trying to hit the ball hard and have good at-bats," Arenado said. "To hit 40 would be an amazing accomplishment. We still have a lot of games left."
Matt Kemp homered for San Diego and has reached base in a career-high 30 consecutive games.
San Diego starter Tyson Ross allowed three runs and four hits in seven innings and also drove in a run with a sacrifice fly in the fourth. He struck out six and walked none.
"I thought Tyson went into the game he wasn't feeling good physically," Padres interim manager Pat Murphy said. "He was sick today. For him to do what he did today to give us a chance to win, I thought he was good. We just didn't take advantage of what we could have under the circumstances."
Ross had given up five home runs in 171 innings before the Rockies tagged him three times. He had gone 45 consecutive starts without allowing multiple homers.
"I think all three of them I was behind in the count," Ross said. "I didn't walk anybody today but in those instances I made mistakes up in the zone and they put good swings on them. That tends to happen. But overall I was happy in how I threw the ball, except for giving up the lead there in the seventh."
Colorado starter Jorge De La Rosa exited after five innings, allowing three runs and six hits.
"Jorge struggled a little bit," Weiss said. "He got himself into a couple jams, but he's really good at limiting the damage. He kept us in position to win."
Rockies: RHP Chad Bettis (6-5, 4.95) will make his third start since coming off the disabled list, looking to improve upon his last outing in which he allowed four runs in 4 1/3 innings as Colorado travels to Seattle. The Mariners will counter with RHP Hisashi Iwakuma (7-3, 4.03).
Padres: RHP Andrew Cashner (5-14, 4.11) takes the mound in San Francisco for the Padres on Friday. Cashner allowed six runs — five earned — over five innings in his last start at Los Angeles. The Giants counter with RHP Jake Peavy (5-6, 4.41).