Just recently had a problem with my Black water system one thing I found was my holding tank vent was plugged!! I used back flowed the vent hose with pressure from the dock by connecting the water hose to the tank end of the vent hose and urge it flushed out some very nasty stuff after it hissed and burgled for a few seconds it was clear it had been plugged.
So morel of the story is check your vents.
My issue was just that nasty black water back up and over pressurization after the tank had over filled, when I opened the deck pump out port nasty black water oozed out like a geyser I also had a hose joint below leak this foul smelling nasty stuff below into the bilge.
Had the vent been clear I would have noticed some stuff spilling onto the stbd deck long before the system had over pressured.
Ed