Carlos Gonzalez led off the ninth with a pinch-hit single against Atlanta closer Arodys Vizcaino (3-3) and Blackmon followed with a two-run shot to right field. It was his 31st homer and matched his career high with 82 RBIs.
LeMahieu followed with a long homer to left, his sixth home run of the season. He had homered in the fifth inning, a long shot to right field.
Atlanta closed the gap in the bottom of the ninth when Matt Adams hit a two-run pinch-hit homer off Colorado closer Greg Holland with one out.
After allowing a hit to Ender Inciarte, Holland was lifted in favor of left-hander Greg McGee, who induced Brandon Phillips into grounding into a game-ending double play. McGee secured his second save.
The winning pitcher was Pat Neshek (4-3 overall, 1-1 with Colorado), who pitched a perfect eighth inning.
The seven runs scored were the most by the Rockies since they had eight against the Milwaukee Brewers on Aug. 18, a span of six games.
Atlanta scored twice in the seventh to tie it 4-4. Ozzie Albies doubled in a run and scored on Dansby Swanson's single to left.
Neither starter received a decision.
Colorado's Kyle Freeland pitched six innings and allowed four runs, 10 hits and one walk, striking out five.
Atlanta's Sean Newcomb departed after 4 2/3 innings. He allowed four runs, seven hits and two walks with five strikeouts.
The bullpens for both teams kept the game close. Colorado got scoreless innings from Chris Rusin and Neshek. Atlanta got 1 1/3 scoreless innings from Matt Wisler and one scoreless inning each from Jose Ramirez and Dan Winkler.
The Rockies took a 1-0 lead in the second inning when Pat Valaika singled home Gerardo Parra, who was hit by a pitch to start the inning.
Atlanta scored twice in the third inning to take a 2-1 lead. Phillips drove in a run with an opposite-field single that hit the first base foul line and Freddie Freeman added a sacrifice fly to left field.
The Rockies scored three runs in the fifth inning to take a 4-2 lead and chase Newcomb. LeMahieu hit a homer, his fifth of the season. Trevor Story knocked in Mark Reynolds and Parra with a two-out double to the gap in left.
NOTES: Atlanta 3B Brandon Phillips extended his hitting streak to 11 games with a first-inning single. ... Atlanta hit into three double plays and leads the National League with 116. ... Colorado 2B DJ LeMahieu ended a streak of 148 consecutive at-bats without a home run when he connected for a solo shot in the fifth inning. His last homer came July 7 against the Chicago White Sox. ... Colorado CF Charlie Blackmon was back in the lineup after missing Friday's game with leg cramps. ... Colorado SS Trevor Story had a stolen base taken away in the eighth inning because C Jonathan Lucroy was charged in interfering with the catcher's throw. ... The finale of the three-game series will be played Sunday. Atlanta RHP Mike Foltynewicz (10-9, 4.95 ERA) will be opposed by Colorado RHP Jon Gray (5-3, 4.54). It is the last meeting between the teams this season. The season series is tied 3-3.