The announcement comes a day after Giants ace Madison Bumgarner had surgery to insert three pins into the broken pinkie on his pitching hand. He is expected to miss at least the first two months of the season.
Blach will become the first Giants pitcher to start Opening Day other than Bumgarner for the first time since Matt Cain got the start to open the 2013 season. Cain's start also was against the Dodgers.
Blach will be opposed by Dodgers ace Clayton Kershaw. Blach has had success at the plate against Kershaw, as he is 3-for-5, including a double.
Blach was 1-1 with spring with a 3.63 ERA in 17 1/3 innings pitched. He was 8-12 with a 4.78 ERA last season. He also replaced Bumgarner in the rotation last season when he missed nearly three months with a shoulder injury after a dirt bike accident.
ESPN Stats & Information contributed to this report.