what is the date on the state heater???
A water heater that is left full of water for a few months
will become stagnant, something like a fish aquarium....
I have seen black gook come out of a water heater that was left full of water for a year or two..
you got to totally flush out that heater to rid yourself of that
perhaps they installed a used water heater that they took out of another unit into your new home
also if the water is from a well you can get a rotten egg smell going....
its best to change the heater