Albert Barnes Commentary 2 Kings 3:23

"and they said, This is blood; the kings are surely destroyed, and they have smitten each man his fellow: now therefore, Moab, to the spoil." — 2 Kings 3:23 (ASV)

The sun had risen with a ruddy light, as is often the case after a storm , nearly over the Israelite camp. The pits, which were deep but had small mouths, gleamed redly through the haze lying along the newly moistened valley, and to the Moabites, they looked like pools of blood.

The preceding year, their enemies and their allies had mutually destroyed each other (2 Chronicles 20:23). Therefore, based on their knowledge of the jealousies between Judah, Israel, and Edom, it seemed to them not unlikely that a similar calamity had now happened to their foes.