The Cardinals selected Blewett's contract from Triple-A Memphis on Friday.
Blewett exercised an opt-out clause in his minor-league contract with the Cards earlier in the week, though St. Louis still held the right to decide whether to add him to their 40-man roster or allow him to walk in free agency. With JoJo Romero (appendicitis) now on the injured list, the team will opt for the former and give Blewett a chance to work with the big club. The 30-year-old righty has put up a 5.18 ERA and a 1.68 WHIP across 48.2 innings at Triple-A this season, so he will likely be limited to low-leverage work in the majors.