Black water is water that is grossly unsanitary and is capable of causing disease. Generally, there will always be Escherichia coli (E. Coli) in sewage waste. It could also contain dangerous chemicals as well as hepatitis, rotavirus and salmonella.
According to the ANSI/IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration, third edition, any water that has its source from beyond the (toilet) trap, regardless of visible color is considered black water.