Green’s Dictionary of Slang

blew v.¹

to inform on, to betray.

1839		implied in blew it
1859, 1860		Hotten Dict. of Modern Sl. etc.
1861		Melbourne Punch ‘The Lay of the Lags’ 14 Mar. 1/1: Then when all the bobbies blewed are, / We can buz and rim a billy, / None to pall the trick and queer us, / Sending us to stun on skilly.
1895		Crowe Aus. Sl. Dict. 10: Blew or Blow, to inform.

In phrases

blew it (v.)

to betray a fellow villain to the police.

1839		H. Brandon Dict. of the Flash or Cant Lang. 161/1: To Blew it – to inform.
1857		‘Ducange Anglicus’ Vulgar Tongue.