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.
In the context of the 1975 season, Blackwell played a reserve role behind Fisk and Bob Montgomery for the Red Sox, who secured their division title. His defensive skills were notable, but his batting average was relatively low at .197. The Red Sox ultimately chose to use Montgomery as their backup catcher during the postseason, which led to 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.