In context, the expression "The night, whose sable breast relieves the stark, / White stars, is no less lovely being dark;" is best interpreted as in the night, we can regenerate ourselves, that although dark, at night, we can go deep in our feelings to born again stronger. At night, "we hid the heart that bleeds, And wait, and tend our agonizing needs."
We use the darkness of the night to clear up our pain and to heal in solitude in order to be reborn when the sunlight comes out.