“This will give the League and American Association from October 20 to November 1 to engage players from the clubs of the minor leagues. The black list of these minor leaguers will be observed.” The Indianapolis News, December 9, 1885, p1
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1897. “In New York these base ball terrors became, for the most part, class D minor leaguers” (The Sporting News, March 27).