Green’s Dictionary of Slang

1. (US) to be a (racetrack) swindler.

1842		Whip & Satirist of NY & Brooklyn (NY) 23 Apr. n.p.: the whip wants to know [...] Whether he blacklegs as much up at Newburgh as he does when he is down here.
1848		Boston Blade 8 July n.p.: You just say they blackleg it once more, if you dare.

2. to work as a strike-breaker [blackleg n.² ].

1888		Baltimore Herald 6 May n.p.: Early this morning the mountain paths leading to the Wiliam Penn colliery were lined with men, dinner in hand, determined to go to work. Some were non-union miners, while the remainder were Knights of Labor who had determined to blackleg it, regardless of the jeers and threats of their companions [F&H].
1892		‘John Miller’ Workingman’s Paradise 29: ‘We can’t trust a lot of girls.’ [...] ‘They’d be ashamed to blackleg if once they got into the union.’.
1897		Worker (Brisbane) 4 Sept. 8/3: But ‘going in’ in time of strike on any terms, to grab, / He doesn’t know as blacklegging but simply calls it ‘scab’.
1901		H. Lawson ‘“Lord Douglas”’ in Roderick (1972) 494: I wouldn’t be surprised if you blacklegged yourself in the end.