De La Rosa (1-3) rediscovered his rhythm, allowing five hits and one run for his first victory of the season. The hard-throwing lefty tied a career high with 16 wins last season.
Ryan Vogelsong (0-1) never got on track and was pulled after 1 1-3 innings, his shortest start with the Giants. He surrendered five runs and six hits, including three homers.
Blackmon lined solo homers in the second and eighth innings. He's now hitting .411 this season.
Arenado, who was moved up to the No. 2 spot in the order, extended his hitting streak to a career-high 12 games.
De La Rosa was on the same page with Rosario all night, something that wasn't the case during the season opener in Miami when the battery mates couldn't get their signals straight. Backup catcher Jordan Pacheco has been behind the plate for De La Rosa's last three starts.
But manager Walt Weiss didn't want to create a situation where De La Rosa had a personal catcher, so he went with Rosario, who actually had a calming influence on the sometimes excitable De La Rosa.
In the third inning, Rosario sauntered out to the mound to briefly chat with De La Rosa after he loaded the bases with no outs. Whatever Rosario said worked as Buster Posey hit into a 6-4-3 double play - bringing in a run - and Michael Morse flew out to left.
The catcher nicknamed ''Baby Bull'' also helped his pitcher out at the plate, too, lining a three-run homer. It's his third homer of the season.
De La Rosa walked off the mound in the fifth to applause after striking out Morse to escape a jam with runners on second and third. Tommy Kahnle took over and struck out five in two innings. It's just the sixth time a reliever has done that in Rockies history.
Colorado pounced on the Giants in the first, with Arenado and Rosario hitting homers off Vogelsong, who threw 44 pitches in the inning.
It didn't get any better in the second when Blackmon hit a solo homer off Vogelsong. The Giants pulled him four batters later.
His replacement, reliever David Huff, bailed the Giants out of more trouble by getting an inning ending double play. Huff strained his left quadriceps muscle running out his first big league hit the next inning and was taken out of the game.
NOTES: OF Angel Pagan was ejected in the seventh for arguing a called third strike. ... Vogelsong's shortest start of his career was Sept. 24, 2004, when he was with Pittsburgh. He faced seven Cincinnati batters and didn't record an out. ... LHP Madison Bumgarner (2-1) will take the mound Tuesday. The Rockies counter with lefty Franklin Morales (1-1). ... Colorado placed OF Michael Cuddyer (strained left hamstring) on the 15-day disabled list and recalled INF Charlie Culberson from the Sky Sox. ... 1B Justin Morneau was named the NL player of the week Monday after hitting .375 with three homers and 12 RBIs.