Blackwater is a term dating to at least the 1970s used to describe wastewater

Wastewater

Wastewater is any water that has been adversely affected in quality by anthropogenic influence. It comprises liquid waste discharged by domestic residences, commercial properties, industry, and/or agriculture and can encompass a wide range of potential contaminants and concentrations...

 containing fecal matter

Feces

Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...

 and urine

Urine

Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...

. It is also known as brown water, foul water, or sewage

Sewage

Sewage is water-carried waste, in solution or suspension, that is intended to be removed from a community. Also known as wastewater, it is more than 99% water and is characterized by volume or rate of flow, physical condition, chemical constituents and the bacteriological organisms that it contains...

. It is distinct from greywater

Greywater

Greywater is wastewater generated from domestic activities such as laundry, dishwashing, and bathing, which can be recycled on-site for uses such as landscape irrigation and constructed wetlands...

 or sullage, the residues of washing processes.

In practice

Water coming from domestic equipment other than toilets (e.g., bathtubs, showers, sinks, washing machines,) is called graywater and it is preferred to be kept separate from blackwater (which comes from toilets) to reduce the amount of water that gets heavily polluted.

Separation of blackwater and greywater nowadays happens with all ecological buildings. Especially in autonomous building

Autonomous building

An autonomous building is a building designed to be operated independently from infrastructural support services such as the electric power grid, gas grid, municipal water systems, sewage treatment systems, storm drains, communication services, and in some cases, public roads.Advocates of...

s, the separation is always present. It is also commonly used on recreational vehicle

Recreational vehicle

Recreational vehicle or RV is, in North America, the usual term for a Motor vehicle or trailer equipped with living space and amenities found in a home.-Features:...

s which feature a graywater holding tank and a blackwater holding tank.

Processing

Blackwater contains pathogens that must decompose before they can be released safely into the environment. It is difficult to process blackwater if it contains a large quantity of excess water, or if it must be processed quickly, because of the high concentrations of organic material.
However, if blackwater does not contain excess water, or if receives primary treatment to de-water, then it is easily processed through composting. The heat produced by naturally occurring thermophilic microorganisms, will heat the compost to over 60 degrees C, and destroy potential pathogens. The compost

Compost

Compost is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming. At its most essential, the process of composting requires simply piling up waste outdoors and waiting for the materials to break down from anywhere...

 is eventually reduced to safe fertilizer after about 1 year.

Elimination

Blackwater can be avoided by making use of composting toilet

Composting toilet

A composting toilet is a dry toilet that using a predominantly aerobic processing system that treats excreta, typically with no water or small volumes of flush water, via composting or managed aerobic decomposition...

s and vermicomposting toilets. In certain autonomous buildings, such as earthship

Earthship

An earthship is a type of passive solar house made of natural and recycled materials. Designed and marketed by Earthship Biotecture of Taos, New Mexico, the homes are primarily constructed to work as autonomous buildings and are generally made of earth-filled tires, using thermal mass...

s, this is almost always present and allows the water requirements of the building (which, with earthships, are self-generated) to be heavily reduced. Besides saving water, composting toilets allow the user to reuse the nutrients found herein (e.g., for growing crops/trees).