Jack Tar is a British slang term for sailors used between the late eighteenth and mid nineteenth century.
The author's use of the name Jack may be a wink at the Jacquerie, a fourteenth century revolt by French peasants which became a term used of all peasant revolts.
French nobility in the pre-revolutionary period often referred dismissively to all peasants as "Jacques". This exacerbated the tensions between the classes that were a cause of the revolts.
In Dicken's A Tale of Two Cities, which is set at the time of the French revolution, three nefarious low born revolutionaries share the name Jacques.
a jack in a deck of cards.
It is also a somewhat old-fashioned word for a deceitful, crafty rogue.
Murder in the Dark is a children's game, played in a darkened room or house.