Jesse Smith's letter to Hyrum Smith refers to "blasphemy" and "iniquity" as "blackness of darkness."
Jesse Smith, Letter to Hyrum Smith, Letterbook 2:59, The Joseph Smith Papers website; accessed on May 21, 2022
Again you say, if you are decieved God is your deciever, Blasphemous wretch— how dare you utter such a sentence, how dare you harbor such a thot— aye, you never did think so, but being hardened in iniquity, you made use of the holy name of Jehovah! for what, why to cover your nefarious designs & impose on the credulity of your Grandfather, one of the oldest men on the earth, Blackness of darkness!