I started following MLB in 1953 at the age of 10. By that time, black players were common. I recall Larry Doby or the Cleveland Indians and Minnie Minoso of the Chicago White Sox. I can't recall black players on the Washington Senators during that period.
Update: Feb. 20, 2017
At First Baptist, Washinton DC, pastor Julie Pennington-Russell treated Jackie's performance as an example of how to turn the other cheek, and the team knew it needed him to do this.