Prying eyes in the LOST garage sale catalog have found MIB's cast chair indicating that the character's name is allegedly SAMUEL.
Another biblical name, for a Hebrew prophet and ruler. In the story, that would make Jacob and Samuel brothers. The oddity is that no one, not even his family members, ever called MIB by his alleged real name.
Man in Black was an attempt to add more mystery to the spirited Smoke Monster. It may go down in LOST lore as another convoluted back story question that will never be truly answered.