We have a state select water heater, model pr650novet. We have flushed it several times but we still have black residue ending up in all our tubs. Does anyone know what else we could do?

Here is one idea: Black particles in hot water - The Tank - Forums - waterheaterrescue.com

What type of supplies do you have on your heater?: Are they braided stainless steel supply lines?
When you flush are the black particles present? Let me know , Tom