Short answers, pulled from the story.
The formal beginning of segregation occurred on October 16 when the Pennsylvania State Convention of Baseball denied admission to the Pythian Baseball Club. This decision marked the start of a policy that would last for decades.
Moses Fleetwood Walker and Weldy Walker were the only black players in major league baseball from 1884 until integration. They both played for the Toledo Blue Stockings in the American Association during that year.
The International League voted on the 14th of July 1887 to ban signing new contracts with black players. A 6-to-4 vote by entirely white teams sent a powerful signal leading to the disappearance of black people from other minor leagues later that century.
Jackie Robinson broke the color line when he signed with the Brooklyn Dodgers in October 1945. He called up to the team after playing one year in the minor leagues with the Montreal Royals of the International League.
The Boston Red Sox were the last major league team to integrate on the 7th of April 1959. General manager Bucky Harris promoted Pumpsie Green from the AAA farm club following a lawsuit charging discrimination and deliberate barring of black players.