The most common cause of a brownish black lump is a common mole. There are also other causes of black lumps. When did you observe this lump or rather has this lump appeared rather recently ? Have you noticed any change in its size/ colour etc ? Any change in a mole requires specialist medical attention such as from a dermatologist. A common mole is called nevus in medical terms. It is harmless. However any change in its size/ texture or other symptoms like itchiness /bleeding etc should not be ignored and should be immediately consulted with a dermatologist who will then do a biopsy. A biopsy is a test where we take tissue, process it and then study it under the microscope to find our if it is a benign or malignant(cancerous) disease process. It is always wise to get a skin check up by a dermatologist especially for fair skinned individuals / sun exposed . It will also allay your fears. My suggestion would be to book an appointment with your dermatologist/ neighborhood GP according to your healthcare setup.