Pitchy means dark and tar like. This word choice reiterates the dark scene in Hell as the angels gather. The scene is described as black and extremely dark. The cloud of locust is compared to night. The comparison implies the idea of Chaos which Milton also compares to night. Milton uses these comparisons to exemplify the dark and evil scene of hell. The cloud of locusts implies the plagues cast into Egypt by Moses to punish the impious Pharaoh. The dark vocabulary set this scene ad carries the theme of religious evil through out the text.