Serious illnesses often develop silently, but the body usually sends subtle warning signs. If any of the six body parts below suddenly turn unusually dark or black, and you don’t know why, it could be a signal that something more serious is happening inside your body.

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1. Black streaks on fingernails or toenails

If a dark brown or black stripe suddenly appears on your nail—especially one running vertically—and it isn’t caused by injury, it may be a sign of subungual melanoma, a dangerous form of skin cancer that grows under the nail.

Warning signs include:

• The dark streak spreading to the skin around the nail (Hutchinson’s sign)

• Uneven color

• Irregular shape

Many people confuse this condition with a simple nail fungus, so early medical check-ups are crucial.

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2. New dark moles or unusual dark patches on the skin

A mole or dark spot that appears suddenly or changes over time can be a sign of skin cancer. The “ABCDE rule” helps identify dangerous moles:

• Asymmetry

• Border irregularity

• Color variation

• Diameter over 6 mm

• Evolving (growing, bleeding, itching, or crusting)

Dark bruises or red-purple spots that appear without injury and don’t go away can be linked to blood disorders, including leukemia, because low platelet levels affect clotting.

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3. Black, tar-like stool

Stool that looks black and sticky like tar (or like coffee grounds) usually indicates bleeding in the upper digestive tract (esophagus, stomach).

While certain foods or medications can also darken stool, persistent black stool without a clear cause may indicate serious gastrointestinal issues and requires prompt medical attention.

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4. Sudden darkening of the face or body

A sudden, unexplained, overall darkening of the skin can be related to metabolic disturbances caused by serious underlying conditions.

In people with severe liver disease, impaired liver function affects melanin and bilirubin metabolism, turning the skin from yellowish to darker. If darkening comes with symptoms like:

• Yellow eyes

• Fatigue

• Weight loss

• Pain in the upper right abdomen

→ you should seek medical care immediately.

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5. Dark, thick, velvety patches on the neck or skin folds

Darkened, thickened skin on the neck, armpits, or groin—known as acanthosis nigricans—can be linked to insulin resistance, obesity, or diabetes.

However, if these patches appear suddenly, spread quickly, or come with weight loss and fatigue, they can also be an early warning sign of internal malignancies (commonly stomach, liver, or pancreatic cancers).

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6. Teeth or gums turning black

Black teeth or dark patches on the gums—especially when not caused by poor hygiene—can be associated with oral disease.

Other warning signs:

• Persistent mouth ulcers

• Gum swelling

• Loose teeth

• Dark, non-healing lesions lasting more than 2–3 weeks

Chronic irritation from smoking, alcohol, or cell abnormalities may contribute to these changes, and early evaluation is important.