It means different things. When it's referring to hair colour (which is probably the nost common sense, historically anyway) it'd mean the sort of jet-black hair colouring you see sometimes, which tends to go with a skin that looks paler, if anything - possibly an optical illusion because of the contrast.

But it'd equally be used in a different sense to refer to a fierce kind of person, not so much a flaring up sort of anger, but rather the unrelenting kind.

And then more recently it's used in the same sense as in Black American or Black British, to mean people with some Afro-Caribbean ancestry.