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.
Blackwell held a reserve position behind Fisk and Bob Montgomery in 1975 when the Red Sox secured their division title. His defensive prowess was evident, but his batting average was limited to .197. The Red Sox ultimately selected Montgomery as their backup catcher for the postseason, which culminated in a successful championship run against the Oakland Athletics in the 1975 American League Championship Series before ultimately losing to the Cincinnati Reds in the 1975 World Series.