From blight + -est. <span class="searchmatch">blightest</span> (archaic) second-person singular simple present indicative of blight...
→OCLC, stanza IV, page 157: Oh, Love! like the blast of the desert thou <span class="searchmatch">blightest</span> / The fairest of flowers with thy venomous breath. 1842, Tertullian, “The...