c. 1300, pur blind "entirely blind," as a noun, "a blind person," later "partially blind, blind in one eye" (late 14c.), the main modern sense, from blind (adj.) + pur "entirely, completely, absolutely."
The adverb forming the first element is from an identical Middle English adjective pur "unadulterated, unmixed" (c. 1300), usually explained as from Old French pur and Latin purus (see pure). It might have been influenced by the Anglo-French perfective prefix pur- (see pur-). The sense of "dull" is recorded from 1530s.