To go with @Greybeard's excellent answer, I quote below some other examples from the OED entry for blot, starting with a very good definition and proceeding to a literal artistic use of it in the sense requested over several centuries.
(A. 1786) A. Cozens New Method in Drawing Landscape 7 "A blot is an assemblage of dark shapes or masses made with ink upon a piece of paper. All the shapes are rude and unmeaning, as they are formed with the swiftest hand. But at the same time there appears a general disposition of these masses, producing one comprehensive form."
(1931) Times 24 Mar. 19/6 "Examples of Cozens's `blots' have long been known. But it was only the other day that five blots accompanied by the five drawings made from them, were discovered."
(1962) Listener 19 July 95/2 "The manipulation of accident in the blot landscapes of Alexander Cozens."
(1910) Edin. Rev. Apr. 371 "Painters are accustomed to speak of the `Blot' of a picture, meaning its immediate appearance as colour, line, massing, or flat space."
(C. 1325) E.E. Allit. P. A. 781 "Vnblemyst I am wyth-outen blot."
All of these refer to a blot as a large smear of ink, obscuring what might lie beneath it, but presenting as a blank. The metaphor of blotting out sin refers to a written account of sin, which can be blotted.
The meanings (literal and metaphoric) are all coherent with the 3 phonosemantic senses of the English assonance BL-: Contained Fluid, Color/Eye, and Excess.