11 “For darkness shall not be darkened by You”. Do not thou then darken your darkness; God darkens it not, but enlightens it yet more; for to Him is said in another Psalm, “You, Lord, shall light my candle: my God shall enlighten my darkness.” But who are they who “darken their darkness,” which God darkens not? Evil men, perverse men; when they sin, verily they are darkness; when they confess not their sins which they have committed but go on to defend them, they “darken their darkness.” Wherefore now if you have sinned you are in darkness, but by confessing your darkness you shall obtain to have your darkness lightened; by defending your darkness, you shall “darken your darkness.” And where will you escape from double darkness, who wast in difficulty in single darkness?...Let us not “darken our darkness” by defending our sins, and “the night shall be light in our delight.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)