Never heard of black water. However, water that is usable but not potable
is usually referred to as grey water. This would be like water from the
sinks, etc. On land this water is usually used for irrigation usage,
usually watering the lawn and maybe washing the car. Therefore I will make
a guess that black water is strictly from the head. If there is a
separation between black and grey water check to see if the 34(36) has three
holding tanks, one for potable water, another for water from the kitchen and
lavatory sinks, and a third for holding crap and stuff.