Blot
late Middle English: probably of Scandinavian origin and related to Old Norse blettr .
wiktionary
From Middle English blot(“blot, spot, stain, blemish”). Perhaps from Old Norse *blettr(“blot, stain”) (only attested in documents from after Old Norse transitioned to Icelandic blettur), or from Old French bloche(“clod of earth”).
etymonline
blot (n.)
late 14c., "a spot or stain of ink;" also "a moral stain or blemish, a disgrace, a sin;" of uncertain origin. Perhaps from Old Norse blettr "blot, stain," or from Old French blot, variant of bloc "block." The Middle English Compendium compares, hesitantly, Old French blo(s)tre, variant of blestre "a boil." From 1570s as "any black or dark patch."
blot (v.)
early 15c., "to make blots (with ink), disfigure with blots," also figurative; mid-15c. "to blot out, obliterate" (words), from blot (n.). Related: Blotted; blotting.