While Jackie Robinson was not technically the first black player in the organization that became the Major Leagues, he broke the color barrier at a time of fierce racial discrimination in all areas of American life.
From Baseball Almanac:
"1878 - Bud Fowler is the first known professional black player on an integrated team when he plays in Lynn (IA) exhibition games."
Soon after these early successes discrimination began to solidify its presence in professional baseball, and a color line was drawn to exclude black players.
Jackie Robinson became the first black player to break the color barrier in the Major Leagues. Robinson first appeared in a Major League Baseball game in 1947. With Robinson's introduction and immediate success, black players began to find widespread acceptance in the American national pastime. Along with fellow pioneers, including greats Don Newcombe and Roy Campanella, Robinson led the Dodgers on one of the most successful runs in baseball history, winning 6 pennants in 10 years. The Dodgers' willingness to oppose social norms of segregation is often cited as one of the primary reasons for their success in the '40s and '50s.