There are a number of skin diseases than can often indicate to the Dermatologist, the presence of an internal cancer/ malignancy. These authors below, report quite a rare entity called Malignant Acanthosis Nigricans. Acanthosis Nigricans is a benign skin condition which occurs commonly is obese individuals, characterised by thick, brown/ black velvety skin change in the armpits.  You can read more about it here: http://www.dermnetnz.org/systemic/acanthosis-nigricans.html.

However if the condition develops suddenly all over the body, one has to consider underlying cancers, as the skin can often develop reactive skin rashes in response to various internal cancers.