Thomas Blackstock was born Kirkaldy, Scotland in 1882. A full-back he played football for Dunniker Rangers, Blue Bell, Raith Athletic, Leith Athletic and Cowdenbeath.
Blackstock did not become a regular in the first team until the 1905-06 season. That year United won promotion to the First Division when they finished second to Bristol City. The club scored 90 goals in 38 games and top scorers were John Picken (20), John Peddie (18) and Charlie Sagar (16).
Blackstock only played in three first-team games in the 1906-07. In April 1907 Blackstock collapsed after heading a ball during a reserve game against St. Helens Town. Frank Buckley, who was standing nearby, helped to carry him to the changing-room. Blackstock died soon afterwards. An inquest into his death returned a verdict of "Natural Causes" but Buckley believed he had died of a heart-attack or seizure.
Several players at Manchester United were angry about the way Blackstock's family was treated after his death. Billy Meredith, Charlie Roberts, Charlie Sagar, Herbert Burgess and Sandy Turnbull, decided to form a new Players' Union. The first meeting was held on 2nd December, 1907, at the Imperial Hotel, Manchester and the Association Football Players Union (AFPU) was established.