From OMIM
Alkaptonuria is an autosomal recessive metabolic disorder characterized by accumulation of homogentisic acid, leading to darkened urine, pigmentation of connective tissue (ochronosis), joint and spine arthritis, and destruction of the cardiac valves (summary by Vilboux et al., 2009). Alkaptonuria enjoys the historic distinction of being one of the first conditions in which mendelian recessive inheritance was proposed (by Garrod, 1902, on the suggestion of Bateson) and of being 1 of the 4 conditions in the charter group of inborn errors of metabolism. The manifestations are urine that turns dark on standing and alkalinization, black ochronotic pigmentation of cartilage and collagenous tissues, and arthritis, especially characteristic in the spine.  http://www.omim.org/entry/203500

From MedlinePlus Genetics
Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air. Ochronosis, a buildup of dark pigment in connective tissues such as cartilage and skin, is also characteristic of the disorder. This blue-black pigmentation usually appears after age 30. People with alkaptonuria typically develop arthritis, particularly in the spine and large joints, beginning in early adulthood. Other features of this condition can include heart problems, kidney stones, and prostate stones.  https://medlineplus.gov/genetics/condition/alkaptonuria