Hi! thanks for your query! This is a sinus tract... After going through ypur description, I would like to explain your problem as follows: it is a deep hole; therefore, the inner end is blind! it is very deep that a pin can go deep down without hurting: therefore, the nerves are either not working or covered by fibrosis/inflammation! It has a black substance in it: therefore, it contains some pigment material like melanin of skin, fungus, cellular debri, old hemorrhage or environmental dirt materials; it has a foul smell: therefore, it is infected or contains inspissated sebaceous secretions Therefore, it is either a ruptured cyst or an infected sinus tract that needs clinical examination and possibly surgical removal; please see a surgeon today. Wishing you good health,