Arenado and Blackmon were voted into the starting lineup by fans. Neither has started an All-Star game before.
To make the playoffs for the first time since 2009, the Rockies will likely at least need to finish with a win total in the high 80s.
This year, LeMahieu's .348/.416/.495 batting, on-base and slugging percentages were all career highs.
Shawn William Scott, 39, is a suspect in the robbery of four banks in Denver in the last two months.