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." Middle English Dictionary compares, hesitantly, Old French blo(s)tre, variant of blestre "a boil." From 1570s as "any black or dark patch."
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.