Blackwell made his major league debut at the age of 21 during a pennant race in July 1974, filling in for an injured Carlton Fisk while the Red Sox were in first place in the American League Eastern Division. Unfortunately, the Red Sox faltered at the end of the season, falling to third place in the final standings. Blackwell was a reserve catcher behind Fisk and Bob Montgomery in 1975 as the Red Sox won the American League Eastern Division title. Although he provided good defensive abilities, he only had a .197 batting average and, the Red Sox elected to use Montgomery as reserve catcher in the post-season as, they defeated the Oakland Athletics in the 1975 American League Championship Series, before losing to the Cincinnati Reds in the 1975 World Series.