He was drafted by the San Francisco Giants in the fifth round of the 2012 Major League Baseball draft. He made his professional debut in 2013 for the San Jose Giants. In 22 games, he went 12–4 with a 2.90 earned run average, 117 strikeouts and 18 base on balls in innings. He spent the 2014 season with the Richmond Flying Squirrels where he was 8-8 with a 3.13 ERA in 25 starts. Blach spent 2015 with the Sacramento River Cats where he compiled an 11-12 record and a 4.46 ERA in 27 starts. The Giants added Blach to their 40-man roster after the 2015 season. Before being called up by the Giants, Blach spent 2016 with Sacramento where he pitched to a 14-7 record with a 3.43 ERA in 26 starts. Blach was promoted to the Major Leagues for September call-ups on September 1, 2016. Blach made his major league debut on September 5, 2016 against the Colorado Rockies. He entered the game in the bottom of the sixth inning and pitched three scoreless innings while allowing one hit. On October 1, 2016, in his second MLB start, Blach pitched eight shutout innings against Clayton Kershaw and the Los Angeles Dodgers, allowing three hits and securing his first win. He also had his first and second MLB hits against Kershaw. Blach was the winning pitcher in Game 3 of the 2016 NLDS against the Chicago Cubs after pitching two scoreless innings in relief. In four games for San Francisco, he was 1-0 with a 1.06 ERA. Despite being a starter throughout his minor league career, Blach opened the 2017 season in the Giants bullpen as their lone left-handed reliever; however, later in April, after pitcher Madison Bumgarner was injured, Blach was put into the starting rotation. On August 3, Blach hit his first major league home run. He spent all of 2017 with the Giants, going 8-12 with a 4.78 ERA in 34 games. He had the fewest strikeouts per 9 innings in the major leagues. Blach started Opening Day of the 2018 MLB season against the Los Angeles Dodgers, his first career Opening Day start. He gave up three hits, three walks, and no runs over five innings to earn the win over Clayton Kershaw. Blach would then for the remainder of the season switch between a starting and relief role for San Francisco. He ended the season with a 6-7 record in 47 games, 13 starts. He struck out 75 batters in innings. In 2019, Blach spent the majority of the first half in the Giants minor league system. He was designated for assignment on July 27, 2019.
Baltimore Orioles
Blach was claimed off waivers by the Baltimore Orioles on August 3, 2019. On September 16, Blach was designated for assignment. On July 15, 2020, Blach underwent Tommy John surgery and would miss the 2020 season.
