A black fingernail often signifies a subungual hematoma—a collection of blood trapped beneath the nail plate. This bruise is a common response to a direct injury, such as a finger being crushed or hit by a hammer. While trauma is the most frequent explanation, identifying the source of the dark color is the necessary first step. The appropriate course of action, ranging from home care to professional medical intervention, depends entirely on this initial assessment.
Identifying the Source of Discoloration
The appearance of the black area helps differentiate between trauma and non-trauma causes. A subungual hematoma appears quickly following an injury, presenting as a dark purple, red, or black patch with clearly defined borders. This discoloration is clotted blood that creates pressure, often causing immediate, throbbing pain.
Discoloration not caused by a recent injury develops slowly over weeks or months. Fungal infections (onychomycosis) can cause a dark appearance, but they typically also result in the nail becoming crumbly, thickened, or yellowed. Rarely, a dark streak can signal subungual melanoma, a type of skin cancer that appears as a vertical band of color.
Immediate Care and Pain Relief
If the black nail results from recent trauma and the pain is mild to moderate, initial management focuses on reducing swelling and internal bleeding. Applying the R.I.C.E. principle—Rest, Ice, Compression, and Elevation—can help minimize the size of the hematoma and limit discomfort. The injured finger should be rested and elevated above the heart to reduce blood flow to the area.
Applying an ice pack wrapped in a thin cloth for up to 20 minutes at a time will constrict blood vessels and decrease swelling. Over-the-counter pain relievers, such as acetaminophen or ibuprofen, can also manage pain and inflammation. This conservative approach is sufficient for smaller, less painful hematomas.
When Professional Drainage is Necessary
Severe, throbbing pain indicates that the blood trapped beneath the nail is creating significant pressure on the sensitive nail bed. If the pain is intense or the hematoma covers more than 25% to 50% of the nail, professional medical drainage is necessary for relief. This procedure is called nail trephination, which quickly alleviates the painful pressure.
During trephination, a healthcare provider creates a small, sterile hole through the nail plate using an electrocautery device or specialized needle. This allows the pooled blood to drain out, often resulting in immediate pain reduction. The procedure is quick and performed under sterile conditions to prevent infection of the nail bed.
Never attempt to drain a subungual hematoma at home using non-sterile tools like a heated paperclip or drill. This practice carries a high risk of introducing bacteria into the nail bed, leading to severe infection. Furthermore, an improperly performed at-home procedure can cause permanent damage to the underlying nail matrix, which grows the new nail.
Discoloration Not Caused by Injury
When black discoloration appears without any history of trauma, immediate medical evaluation is warranted. Fungal infections (onychomycosis) can cause dark streaks or patches due to pigmented fungi or debris trapped under the nail. Unlike a hematoma, a fungal infection often causes nail thickening, lifting away from the nail bed, and a crumbly texture.
These infections require prescription-strength oral or topical anti-fungal medications, as simple drainage is ineffective. A dark, vertical streak that widens over time, has irregular borders, or extends into the cuticle or surrounding skin is a red flag for subungual melanoma. This rare but serious cancer, unlike a bruise, will not grow out with the nail and requires immediate diagnosis by a dermatologist through a biopsy.
Monitoring and Long-Term Nail Regrowth
For a black fingernail caused by a hematoma, the long-term solution is simply time. The damaged nail will remain dark as it is slowly pushed forward by the growth of a new, healthy nail underneath. Fingernails typically grow out completely in about six to nine months.
As the new nail grows, the old, discolored nail may eventually lift away from the nail bed and fall off entirely. If the old nail detaches, the newly exposed nail bed should be kept clean and protected with a sterile bandage until the new nail hardens. Monitor the new growth for any persistent discoloration, abnormal texture, or unusual ridges, which could indicate permanent damage to the nail matrix or an underlying issue requiring further medical attention.