Jackie Robinson never played for the Red Sox, though on April 16, 1945 the team held tryouts at Fenway Park for Jackie Robinson and other black players. The tryouts were not legitimate, and serve a stark reminder that the Boston Red Sox weren't at the forefront of the integration of black players. Robinson signed with the Brooklyn Dodgers in August of 1945, and he debuted on April 15, 1947. The Red Sox would not add a black player to their roster until July, 1959.
The Washington Nationals/Zombie Expos didn't exist until 1969, but the franchise has historically been well represented by diversity, including drafting Andre Dawson, one of the best Montreal Expos of all time and a Hall of Famer. Dawson played two seasons for the Red Sox as well, but he was pretty much washed up by then.
In 1997, MLB retired Jackie Robinson's #42, and beginning in 2009 all major league players and coaches began wearing #42 on Jackie Robinson Day. Two years before then Ken Griffey Jr. asked Rachel Robinson permission to wear #42, and it quickly spread. So while Jackie Robinson played only for the Brooklyn Dodgers, all of Major League Baseball honors Jackie Robinson on April 15.
The 2015 Red Sox season, though young, has so far been a break out for Mookie Betts. Named Markus Lynn Betts by his parents partly because he would share the initials with MLB, Betts has given hope to major league baseball's issue of not having African Americans represented more fully at the major league level. The hope here is he'll hit a few bombs, steal a few bases, and rob someone else of a home run in the finale versus the Nationals!