From Starbounder - Starbound Wiki
Dark Shirt is an object unobtainable by players through normal means. This item's information is present in the game files, but not intended by developers to be in players' possession.
Dark Shirt is the chest piece of the Dark Set, a costume armor set worn by the Shadow race. It emits a faint veil of smoke when worn.
This shirt is not to be confused with the Dark Disguise Shirt, which is obtainable by the player and has a slightly less dark appearance.
- Glad Giraffe: Added