BLUE SKIN. A person begotten on a black woman by a white man. ["]One of the blue squadron["]; any one having a cross of the black breed, or, as it is termed, a lick of the tar brush.
The equation of having dark skin with blueness may be widespread: the Dictionary of American Regional English finds that black people themselves called very dark-skinned blacks "blues"; while Eric Partridge's Dictionary of Slang and Unconventional English finds half-black, half-white people are called "blue skins" in the West Indies. The Janjaweed in the Sudan call the black African Muslims they terrorize zurqa, from Arabic azraq, "blue." (Someone on Wikipedia believes that this use of "blue" is actually a polite one.)
By the way, blue can mean dirty or sexual, as in "blue movie"; or drunk, in both English and German; among many other things.