Check whether the particles appear at one faucet or the whole house, hot or cold, and whether they smear like rubber or crumble like carbon. That usually narrows the source quickly.
Carbon fines from a filter
Municipal water guidance notes that granular activated carbon can appear as black coffee-ground-like particles when filter media escapes or a new cartridge sheds fines.
Flush a new filter according to the manufacturer instructions and inspect damaged housings or cartridges if the issue persists.
Rubber gaskets and hoses
Degrading rubber can produce oily or flexible black particles, often localized to one faucet, appliance hose or fixture.
Manganese and mineral particles
Groundwater and utility conditions can introduce dark mineral deposits. If black particles occur throughout the home and are not associated with a filter or hose, water chemistry deserves consideration.
When testing or service makes sense
A persistent whole-house issue should be investigated before adding another filter. Tap testing can identify manganese and other chemistry, while a plumber can inspect deteriorating components.