The collapse of a mighty empire is not merely a local political event. It is a profound shock, feeling as though the very systems of nature are breaking down and all light is giving way to total darkness [שטיינזלץ].
The primary approach among commentators views this imagery through a powerful metaphor. Pharaoh, the king of Egypt, is compared to a massive, brightly burning candle that represents his immense success and greatness. Naturally, when a very large candle is snuffed out, it releases thick, heavy smoke. In the same way, when God extinguishes Pharaoh's reign, the resulting ruin will be like a thick smoke rising to cover the sky and block out the light of the heavens [רש"י, רד"ק, מצודת דוד, אברבנאל].
The darkening of the sun and moon operates on several levels. On a psychological level, it paints a vivid picture of extreme distress. A person suffering through such profound tragedy feels as if the entire world around them has gone completely dark [רד"ק, מצודת דוד, אברבנאל]. Furthermore, this darkness represents the intense mourning and shock of neighboring nations. Watching Egypt fall, these nations will tremble in fear for their own survival as destruction approaches their borders [רש"י].
Another layer of this imagery connects directly to the beliefs of the Egyptians themselves. The land of Egypt was known for its bright, cloudless skies, and its people believed their guiding star was the supreme, undimming king of all stars. For them, a cloud covering the sun was the ultimate symbol of failure and decline. Using their own cultural symbols, the prophecy illustrates how their light will be completely snuffed out, representing the total loss of all their allies and supporters [מלבי"ם].
Offering a different perspective, one approach moves away from viewing the darkness merely as a metaphor. Instead, it sees a description of God directing the world through the heavens. According to this view, the sun, moon, and stars guide the fates and successes of earthly kings. When God decrees the end of Pharaoh's rule, these heavenly bodies literally darken in the sense that they stop shining their favorable influence and abundance upon Egypt. Without this heavenly support, the kingdom inevitably sinks into the darkness of absolute ruin [אברבנאל].