Quick take
Test for manganese, iron and sulfur before buying. Whole-house oxidation and media filtration are common when manganese is confirmed.
Manganese is a common cause
Manganese can create dark-brown or black deposits in fixtures and plumbing.
Sulfur chemistry can complicate the picture
Hydrogen sulfide and metal sulfides can also create dark deposits, particularly in well-water systems.
Localized black debris may be plumbing
Rubber washers, hoses or filter media can produce black particles at only one fixture.
Treatment path
A whole-house well-water system should be selected from measured chemistry rather than stain color alone.