Black particles most often come from a water filter, a degraded faucet washer or gasket, or from a deteriorated flexible supply hose. If the particles look similar to coffee grounds, they are probably granular activated carbon from inside a water filter. This is easily fixed by replacing the filter cartridge. If the problem persists, consult with the manufacturer of the unit or a water systems professional to make sure a bigger problem is not to blame. If the particles are small black specks that look oily or sooty, they are probably from a black rubber flexible hose. Over time, the chlorine in the water breaks down the rubber. To fix this problem, simply replace the hose with one that has a protective lining.