"swindler," originally especially in equestrian events, 1771, from black (adj.) + leg (n.), but the exact signification is uncertain.
The term implies the habitual frequenting of places where wagers are made and games of chance are played, and the seeking of subsistence by dishonorable betting, but does not always imply direct cheating. Sometimes contracted to leg. [Century Dictionary]
Used from 1865 of strike-breakers and workmen who refused to join trade unions.