«فَلَاحَ لَيْلٌ عَلَى صُبْحٍ»
Then night appeared above the morning.
Here night represents dark hair and morning the bright face beneath it. The words retain their familiar natural associations, but serve as metaphors for appearance.
The contrast is central to the image: darkness frames and intensifies the impression of brightness. The subject is a person, yet the poet borrows the language of the natural world to make that person vividly present.