Water from a washing machine, kitchen sink, and shower is typically considered gray water. However, water from the toilet is classified as black water due to the presence of harmful pathogens and waste.
So, in your question, the source that would be technically considered gray water but should be treated as black water is the water from a kitchen sink. This is because it can contain food particles, grease, and other contaminants that may need more careful treatment compared to typical gray water sources.