I always understood that "black Irish" was used in relation to hair colour, creamy white skin and blue eyes as distinct from the red-haired, usually green-eyed and freckled skin folk. "Dubh" in Irish when used in relation to people usually means "dark-haired" as in the songs "Bean Dubh a' Gleanna" (the Dark-Haired Woman of the Glen) and "An Buachaill Caol Dubh" (the Dark-Haired slender boy). Likewise "Rua" rather "dearg" refers to "red-haired" as in "Bean an Fir Rua" (The Red-Haired Man's Wife).