Black cyst under the skin: causes and warning signs

A true cyst sits beneath the surface and contains keratin or fluid within a lining. The overlying skin may remain normal, show a dark central pore, or darken after inflammation; colour does not confirm what lies underneath.

Pressure, friction and inflammation can stimulate pigment change, especially in deeper skin tones. Bruising after squeezing may look purple-brown or black, while a tiny dark pore can mark the channel from an epidermoid cyst to the surface.

A soft mobile lump behaves differently from a hard fixed one, but touch alone cannot establish the diagnosis. Depth, growth, tenderness, warmth, drainage, skin breakdown and the duration of colour change all guide the clinical assessment.

Avoid pressure from tight clothing or straps, cleanse normally and stop squeezing or massaging the lump. Photograph it in consistent lighting and note whether the outline, colour, tenderness or mobility changes.

Do not use a needle or make an incision at home. Deep lumps may involve structures that are not visible, and incomplete drainage can worsen inflammation. Treatment may involve watchful waiting, calming inflammation, clinician-performed drainage or removal of the entire cyst lining.

Obtain urgent assessment for rapidly worsening pain or swelling, fever, spreading redness, pus, blistering, a black area that is expanding, loss of feeling, cold surrounding skin, or a lump that appeared after a significant injury.

Schedule a timely skin review when a deep lump enlarges, becomes fixed, returns after previous drainage, causes persistent discomfort, breaks the surface, bleeds unexpectedly or has new uneven pigmentation. Imaging or sampling is sometimes used when examination cannot establish its nature.

Ask whether the lump appears cystic or solid, whether its depth warrants imaging, what explains the skin colour, whether inflammation changes the timing of a procedure, and which option addresses both current symptoms and recurrence.