I have done some water heater service work for a few year's and haven't run into any black particles only white from deteriating dip tube.
no idea if it helps but interesting.
Black particles can come from four common sources:
•The inside of a steel pipe,
•A broken water filter,
•A degrading faucet washer or gasket, or
•A disintegrating, black rubber, flexible supply line hose (these same hoses are also made as flexible water heater connectors).
Particles from the inside of a steel pipe are discussed in more detail under the brown or orange particles section.
If the particles are very hard, similar in size and shape, and might be described as large coffee grounds, they are probably granular activated carbon (GAC) from the inside of a GAC water filter. To stop this problem, replace the filter cartridge or consult with the manufacturer of the unit or the salesperson who sold it to you. If the particles are small black specks that might be described as being oily or sooty in texture, they are probably from the inside of a flexible hose. Over time, the chlorine or chloramine in the water causes the rubber to break down. To stop this problem replace the hose with one of the new styles that have a water disinfection resistant lining (clearly advertised on the label) or change to a different style of hose that is not made of black rubber.