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 arthritis, kidney stones, spots of dark pigmentation, and dark urine. Ochronosis, a buildup of dark (blue-black) pigment in certain tissues, is a characteristic feature of alkaptonuria. The first symptom of alkaptonuria is often urine that turns black or very dark when it is exposed to air (oxidation). However, this color change may not occur immediately after urination.

Ochronosis occurs in connective tissues throughout the body, such as the joints, tendons, and ligaments. Pigment changes can be easily seen in the whites of the eyes (sclera), the outer ears, and the hands. Dark pigment can also be found in earwax and in body sweat (perspiration). These pigment changes are usually evident after age 30.

Ochronosis in the joints causes people with alkaptonuria to develop arthritis, typically in early adulthood. Arthritis usually affects the spine and large joints (such as the hips and knees). Many affected individuals require joint replacements later in life.

Over time, the deposits of pigment can harden (calcify). Calcification in joints or ligaments causes them to become rigid and brittle, decreasing their flexibility and making them prone to damage. Calcification of the discs that separate the bones of the spine can cause further back pain. This calcification can also affect the cartilage that makes up the valves of the heart, which allow blood to move through the heart and prevent blood from flowing backward. As a result, the heart valves narrow (stenosis), which can cause the valves to leak (regurgitation).

The body removes the excess pigment in urine. However, removing the large pigment deposits found in people with alkaptonuria can put strain on the kidneys, and this can lead to kidney (renal) failure. Similarly, a buildup of these substances in the kidneys or prostate gland can cause calcium stones to develop. If the body cannot effectively process and remove the pigment and related substances, these products substances can continue to build up in the body, worsening ochronosis, arthritis, and other features of alkaptonuria.

With appropriate medical management, people with alkaptonuria generally have a normal life expectancy.  https://medlineplus.gov/genetics/condition/alkaptonuria