blackhead
Specifically refers to a blackhead (comedone) on the skin, often in skincare contexts.
I'm bothered by the blackheads on my nose.
dark discoloration; blackening; tarnish
General term for dark staining or discoloration on surfaces, metal tarnish, or soot-like marks.
Dark tarnish has appeared on the silver accessory.
The dark discoloration on the wall might be mold.
Noun form of the verb 黒ずむ (to blacken), from 黒 (black) + ずむ (a suffix indicating a state or tendency).