What is the medical term for a darkly pigmented cancerous tumor?

Yes, you may be looking for the word "melanoma" - a cancer of the

Melanin is actually the the name given to a group of pigments that determine the color of our skin and hair. And the cells where these pigments are created, the melanocytes, can be susceptible to cancer.

Therefore, melanoma is usually a form of skin cancer, as this is where most of the body's melanin is created.

However, melanin also lines many of our internal organs, and our eyes too... So it is technically possible to suffer from melanoma in other parts of the body other than the skin.