Green’s Dictionary of Slang

(later US black) the police.

1886		Sporting Times 13 Feb. 1/4: ‘Look here,’ said Little Boy Blue 271E, as he moved the trio on.
1890		Sporting Times 11 Jan. 1: ‘I’m off,’ yelled the suddenly aroused Biscuit, thinking it was Little Boy Blue being announced.
1941		D. Burley N.Y. Amsterdam Star-News 17 May 11: The head chick spieled that it might be little boy blue.
1980		E. Folb Runnin’ Down Some Lines 66: The police are characterized in terms of the uniforms and the colors that mark them as the legal arm of the establishment (blue coats, blue boys, little boy blue).
1995		L. Stavsky et al. A2Z.