Thanks for the query to H.C.M. Forum.
As you mentioned in your query that there is mole extending from root of nail to end.
In my opinion as described you this is not a mole . May be ,
, diagnosis can be confirmed by physical examination by a dermatologist .
2 May be a bacterial or fungus infection . Diagnosis can be confirmed by physical examination by a dermatologist.
3 May be due to impede blood supply as color is dark.
In my opinion either upload the picture of this part or consult a dermatologist
/ surgeon and get their opinion.
may be the cause if chronic and in old age as you didn't mention these things in query.
Good luck. Dr. HET