Is the blackest mantra of the twenty-four filial piety pictures a rhetorical device?

Yes It means to curse all those who oppose and hinder the vernacular with the blackest, blackest and blackest mantras. "The three blackest adjectives fully express Lu Xun's deep-rooted hatred of these feudal remnants. They have the meaning of strengthening layer by layer in tone, and the parallelism form uses repeated rhetorical devices.