Green’s Dictionary of Slang

blue adj.6

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

black, as in skin colour.

[Aus][A. Harris] (con. 1820s) Settlers & Convicts 163: Billy Blue; so called, I suppose, because he was a very black black.
[US]Elman & Handy ‘Blue Gummed Blues’ [lyrics] He’s an eight rock, an’ I’d shock my people if they knew That my man’s gums are blue.
[US]Van Vechten Nigger Heaven 157: I’m too blue for that pink-chaser.
[US]Z.N. Hurston Mules and Men (1995) 70: Blue Baby stuck in his oar and said: ‘He ain’t so ugly.’.
[US]H. Rhodes Chosen Few (1966) 29: Boy, you so black, you blue.
[US]M. Braly False Starts 155: Across the mess hall we can see the blue Quentin population [...] The blacks were segregated in the mess hall.
[US]J.L. Dillard Lex. Black Eng. 74: Blue is still widely used among Negroes in the meaning ‘extremely dark’.

In compounds

blue-black (adj.)

(US black) of skin colour, so dark it seems to have tints of blue.

[US]C. McKay Home to Harlem 47: One iron-heavy, blue-black lad [...] carried his arm in a sling.
[US](con. WWI) H. Odum Wings on My Feet 281: Some big black boys look like blue black, some brown like copper statue.
blue boy (n.)

see separate entry.

blueskin (n.)

see separate entry.

In phrases