Literal meaning is about weather, but equally common figuratively for looming crisis or trouble - 'dark clouds gathering over' a company, relationship, or political situation. Pairs often with 风雨欲来 (storm about to come).
memory
密布 ('densely spread') is the engine - it's not just clouds (乌云), they're packed across the sky. Break it down: 乌 (black) + 云 (cloud) + 密 (dense) + 布 (spread).
