Blackwater is a term dating to at least the 1970s used to describe wastewater containing fecal matter and urine. It is also known as foul water, or sewage. It is distinct from greywater or sullage, the residues of washing processes.
Water coming from domestic equipment other than toilets (e.g., bathtubs, showers, sinks, washing machines) is called greywater 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 buildings, the separation is always present. It is also commonly used on recreational vehicles which feature a greywater holding tank and a blackwater holding tank.
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 it 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 °C (140 °F), and destroy potential pathogens. The compost is eventually reduced to safe fertilizer after about 1 year. 
Blackwater can be avoided by making use of composting toilets and vermicomposting toilets. In certain autonomous buildings, such as earthships, 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 therein (e.g., for growing crops/trees).