Green’s Dictionary of Slang

blues n.2

[the colour]

1. [mid-19C+] the police.

2. [1910s+] (Aus./US) a blue uniform.

3. [1940s] any trousers.

4. [1940s] a police uniform.

5. [1960s+] (US prison) a prison uniform.

6. [1960s+] amphetamines.

7. [1970s] (US) a sailor’s trousers.

8. [1970s+] (US) blue jeans.

9. [1980s+] (Aus. prison) prison officers; thus used as adj. to refer to staff activity, e.g. blue talk, prison officer conversations etc.