Green’s Dictionary of Slang

1. (US Und.) an act of departure.

1909

H. Green Mr. Jackson 97: We’re goin’ to do this blow to ‘Chi’ in the mornin’.

2. (US police) the end of a work shift.

1973

E.F. Droge Patolman 69: I didn’t mind the peaceful demonstrations so much because if the crowd broke up before 4P.M. there was a good chance we would get an ‘early blow’ .