Green’s Dictionary of Slang

corrupt, underhand.

1851		R.S. Surtees Young Tom Hall 9: Colonel Blunt [...] a great, coarse, blackleg sort of man.
1934		R.L Bellem ‘The Shanghai Jester’ in Spicy Adventure Stories July 🌐 The Argus Agency was a blackleg crowd who wouldn’t hesitate to [...] slip a knife between his ribs.