Black dot on/under fingernail. Is it melanoma?

Unfortunately your photograph did not upload but in any case its worth going to your GP for an assessment given the length of time the "dot" has been present.  If necessary your GP can refer you to a specialist if further treatment is required.

It is hard to assess without a picture but if the mole looks
dark and/or irregular and if it appeared suddenly or changed recently, it is
best to get it looked at by your dermatologist and determine if a biopsy is
needed. In addiiton, if there are any worrisome features or symptoms like
itching, burning, pain or bleeding, do not hesitate and have it removed sooner
than later. A consultation from your dermatologist and possible biopsy would
give you a definitive answer if it is benign or cancerous and follow-up would
be determined based on the result.

"This answer has been solicited without seeing this
patient and cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained dermatologist for appropriate care."