A dark discoloration beneath a toenail can be unsettling. The black color signifies a change in the tissue or material under the nail plate, often indicating the presence of blood, pigment, or infectious debris. While the appearance of a black toenail requires attention, it is most often the result of a common, non-serious injury. Distinguishing the cause determines if the discoloration will simply grow out or if it signals a more significant health concern. Causes generally fall into three categories: mechanical trauma, infectious agents, or pigment production within the nail unit.
Blood Under the Nail Plate (Subungual Hematoma)
The most frequent cause of a black toenail is a subungual hematoma, essentially a bruise forming beneath the nail plate. This condition results when blood vessels of the nail bed rupture, trapping blood between the bed and the hard nail plate. The initial pooling causes a red or purple appearance that quickly transitions to dark brown or black as the blood clots and breaks down.
The injury can occur from a single, acute event, such as stubbing a toe or dropping a heavy object on the foot. More commonly, the hematoma develops from repetitive microtrauma, especially in athletes like runners or hikers. This stress happens when the toes repeatedly slide forward and impact the inside of a shoe’s toe box, causing continuous damage to the nail bed.
The trapped blood creates pressure that often results in a painful, throbbing sensation. This pressure frequently subsides within a few days as the blood congeals, but the discoloration remains fixed beneath the nail. The dark area gradually migrates forward as the nail grows out, a process that can take many months. In cases of severe injury, the pressure can cause the nail plate to separate, leading to eventual shedding.
Fungal and Bacterial Causes of Discoloration
Infectious agents can also lead to a dark appearance, distinct from a trauma-induced bruise. Onychomycosis, a common fungal infection, can result in black discoloration in several ways. The fungus may cause the nail plate to thicken and crumble, leading to a dark color from debris accumulation beneath the nail.
Certain types of mold, such as Scytalidium species, are pigmented fungi that directly produce a brown or black color within the nail structure. Fungal infections often present with features like nail thickening, brittleness, or a foul odor, and they typically do not cause the acute pain associated with a hematoma.
A different type of infection, caused by the bacterium Pseudomonas aeruginosa, results in a distinct greenish-black hue known as chloronychia or green nail syndrome. This bacterium thrives in moist environments, often colonizing the space beneath a partially separated nail. Pseudomonas produces a blue-green pigment called pyocyanin, which stains the nail plate. The discoloration is typically localized and often occurs without significant trauma.
Pigmentation and Serious Underlying Conditions
A black toenail may be caused by the production of melanin, the pigment that colors skin and hair. This manifests as a dark streak running vertically from the cuticle toward the tip of the nail, called longitudinal melanonychia. This is a common and benign finding in individuals with darker skin tones, resulting from increased melanin production by the matrix cells.
However, any new or changing dark streak must be evaluated because it may represent subungual melanoma, a rare but aggressive form of skin cancer. Melanoma typically begins as a longitudinal band that may change color, widen, or have irregular borders. A concerning sign is Hutchinson’s sign, where pigmentation extends from the nail plate onto the surrounding skin of the cuticle or nail folds. This spreading of pigment beyond the nail is an important clinical indicator.
Unlike a trauma-induced hematoma, the discoloration from melanoma will not grow out with the nail in a uniform, forward-moving band. Systemic factors can also cause generalized nail darkening, including some medications used in chemotherapy or antimalarial treatments.
Identifying Red Flags and Seeking Care
While most black toenails are minor injuries that resolve themselves, a professional evaluation is warranted if “red flags” are present. If the discoloration appeared without known trauma, or if the cause remains unclear, consultation with a dermatologist or podiatrist is recommended.
Specific symptoms that signal the need for medical attention include signs of infection, such as increasing pain, swelling, warmth around the nail, or the presence of pus or foul odor. A rapidly changing dark area, or one that covers more than half of the nail, should be examined promptly.
The presence of Hutchinson’s sign—pigmentation extending onto the skin surrounding the nail—is a serious indicator that requires immediate investigation to rule out melanoma. If the dark area does not move forward with the natural growth of the nail over several months, this suggests a persistent issue in the nail matrix rather than a trapped blood clot.