Black residue from faucets is usually caused by deteriorating rubber washers or gaskets inside the faucet or supply lines. Manganese in the water supply can also produce black deposits. Activated carbon from water filters may release particles when new. Clean the aerator by unscrewing it and rinsing. Replace worn internal faucet components. If manganese is the cause install a water treatment system. Black residue is generally not harmful but indicates components need maintenance or replacement.