Black Patches on Soles of Feet: Causes, Diagnosis, and When to Worry

Noticing a new black patch on the sole of your foot can be unsettling. While your mind might jump to the worst-case scenario, it's important to know that the causes for these spots range from completely benign to serious medical conditions that require prompt attention. Understanding the potential reasons and knowing what to look for is the first step in protecting your health.

This comprehensive guide synthesizes medical information, expert advice, and real-world experiences to help you navigate what a black patch on your foot might mean and when you should seek professional medical advice.

Common and Benign Causes of Black Patches on Feet

Many dark spots on the feet are harmless and result from everyday life or minor skin conditions.

Trauma and Bruising

The simplest explanation is often trauma. Stepping on a small, sharp object or repeated impact from running can cause tiny blood vessels under the skin to break, leading to a bruise or a blood blister (subungual hematoma) that appears as a black, blue, or purple patch. These spots are typically tender at first and will gradually fade and disappear as the body heals.

Hyperpigmentation due to Friction

Constant pressure and friction from ill-fitting shoes can cause the skin on your soles to thicken and produce excess melanin, the pigment that gives skin its color. This can lead to dark patches, especially over calluses or pressure points. This is a form of post-inflammatory hyperpigmentation and is generally not a cause for concern, though it may indicate a need for better footwear.

An image of a brown or black patch caused by tinea nigra on the sole of a foot. Image Source: Medical News Today

Fungal Infections (Tinea Nigra)

Tinea nigra is a rare, superficial fungal infection caused by the yeast Hortaea werneckii. It presents as a painless, flat, brown or black patch that looks like a stain on the skin. It's often mistaken for a more serious condition.

• Appearance: Typically a single, slow-growing patch with a well-defined border. It is not scaly or itchy.
• Causes: The fungus thrives in warm, humid environments and can enter the skin through a small cut. It's more common in tropical or subtropical regions.
• Treatment: It is easily treated with topical antifungal creams prescribed by a doctor.

Plantar Warts

Plantar warts are common growths on the soles of the feet caused by the human papillomavirus (HPV). While the wart itself is usually skin-colored, it often contains tiny black dots. These are not seeds, but rather small, clotted blood vessels, which can sometimes give the wart a dark overall appearance.

Serious Causes Requiring Medical Attention

While less common, a black patch can be a sign of a serious underlying medical condition. Early diagnosis is critical for effective treatment.

Acral Lentiginous Melanoma (ALM)

This is the most serious potential cause of a black patch on the sole of the foot. Acral lentiginous melanoma (ALM) is a rare but aggressive form of skin cancer that appears on the palms of the hands, soles of the feet, or under the nails.

Demographic Disparities and Diagnostic Challenges

Unlike other melanomas linked to sun exposure, ALM is not. It is the most common type of melanoma in people with darker skin tones, including individuals of African, Asian, and Hispanic descent. The tragic death of musician Bob Marley at age 36 from ALM that began under his toenail brought global attention to this disease.

Unfortunately, diagnosis is often delayed in people of color, leading to poorer outcomes. This is partly due to a lack of awareness and a scarcity of images of ALM on dark skin in medical textbooks and public health campaigns. As college football player Joshua Paschal, who was diagnosed with ALM on his foot, told TODAY.com, he initially dismissed the spot as a blood blister.

How to Spot Potential Melanoma: ABCDE vs. CUBED

Dermatologists use two acronyms to identify suspicious lesions.

The ABCDEs of Melanoma:

• Asymmetry: One half of the spot does not match the other.
• Border: The edges are irregular, ragged, notched, or blurred.
• Color: The color is not uniform and may include shades of black, brown, pink, red, white, or blue.
• Diameter: The spot is larger than 6 millimeters across (about the size of a pencil eraser), although melanomas can be smaller when first detected.
• Evolving: The mole is changing in size, shape, or color.

An illustration comparing a normal mole to a cancerous melanoma using the ABCDE guidelines. Image Source: Skin Cancer Foundation

Because ALM can present differently, podiatrists and dermatologists developed the CUBED guide specifically for lesions on the feet:

• Colored: The lesion is a different color than the surrounding skin.
• Uncertain: The diagnosis is unclear.
• Bleeding: The lesion is bleeding or oozing, even under a nail.
• Enlargement: The lesion or ulcer is getting bigger or not improving with treatment.
• Delay: A lesion takes more than two months to heal.

Vascular and Systemic Conditions

Poor circulation can cause skin discoloration on the lower extremities.

Venous Stasis Dermatitis

According to the Center for Vascular Medicine, this condition occurs when faulty valves in leg veins allow blood to pool and leak into the surrounding skin. This can cause reddish-brown, purple, or dark patches, along with swelling, itching, and thickened skin, primarily around the ankles and shins but sometimes affecting the feet. It is a sign of underlying venous insufficiency.

Diabetic Foot Complications

People with diabetes are at high risk for foot problems due to poor circulation and nerve damage (neuropathy). A black patch could be a sign of a developing foot ulcer or, in severe cases, gangrene (tissue death) from a lack of blood flow. Any person with diabetes who notices a new dark spot on their foot should seek immediate medical care.

When to See a Doctor or Podiatrist

Given the possibility of a serious condition like melanoma, a "wait and see" approach is not recommended. You should schedule an appointment with a dermatologist or podiatrist if you notice a spot that:

• Is new and has no obvious cause (like a known injury).
• Is changing in size, shape, texture, or color.
• Matches any of the ABCDE or CUBED warning signs.
• Is painful, itchy, tender, or bleeding.
• Is an open sore that won't heal.
• You have a personal or family history of skin cancer.
• You have an underlying condition like diabetes or peripheral artery disease.

The most important rule is: when in doubt, get it checked out.

Diagnosis and Treatment

A doctor will start with a physical examination of the spot. They may use a dermatoscope, a special magnifying lens, to see the lesion more clearly.

The only way to definitively diagnose melanoma is with a skin biopsy. This is a simple procedure where a small sample of the tissue is removed and sent to a lab for analysis.

• Benign Patches: If the spot is benign, treatment will depend on the cause. Bruises will heal on their own. Hyperpigmentation can be addressed with better footwear. Fungal infections are treated with antifungal medication.
• Serious Conditions: If the diagnosis is melanoma, a team of specialists will determine the best course of action, which may include surgery, immunotherapy, or other cancer treatments. For vascular conditions or diabetic complications, a doctor will work to manage the underlying disease.

References

• Center for Vascular Medicine - What Are Dark Spots on Bottom or Top of Feet?
• Medical News Today - Tinea nigra: Diagnosis, treatment, and remedies
• Fox Vein & Laser Experts - What Causes Brown & Dark Spots on Bottom of Feet?
• TODAY.com - Small dot turned out to be deadly melanoma hiding on the sole of his foot
• Consumer Reports - For Black People, Surviving Skin Cancer Begins With Knowing What to Look For—and Where
• American Academy of Dermatology - Signs that could be melanoma on your foot