Blackening of urine, when exposed to air is a metabolic disorder in human beings. This is due to
Alkaptonuria (black urine disease, black bone disease, or alkaptonuria) is a rare inherited genetic disorder in which the body cannot process the amino acids phenylalanine and tyrosine, which occur in protein. Homogentisic acid and its oxidised form alkapton are excreted in the urine, giving it an unusually dark colour.