I don’t know, since my knowledge of Western occultism is dogshit, but if Borderlands WERE a more general use term (and I suspect it was), I would go looking for it in the spiritualism stuff of the time; they were among Craddock’s influences. Schmidt mentions in his bibliography Ann Braude’s book, “Radical Spirits: Spiritualism and Women’s Rights in Nineteenth-Century America,” which I’ve been meaning to check out! It might be a good place to start.