Green’s Dictionary of Slang

blue adj.6

[blueskin n. (2) + see combs. below for earlier uses]

[early 19C; 1920s+] black, as in skin colour.

In compounds

blue-black (adj.)

[late 19C+] (US black) of skin colour, so dark it seems to have tints of blue.

blue boy (n.)

see separate entry.

blueskin (n.)

see separate entry.

In phrases

one of the blue squadron (n.) [blueskin n. (2a)]

[late 18C–early 19C] a person of mixed race.