A gambler or sharper on the turf or in the cockpit: so called, perhaps, from their appearing generally in boots; or else from game-cocks whose legs are always black.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Black Indies * Black MondayNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes