The Blue Gown of the Sullen Moon was a full-length gown, primarily blue in hue, that reached down to the floor. Its design incorporated long, wide sleeves featuring a yellow material as the inner lining. Additionally, it included a panel of yellow textile situated underneath a pair of smaller, blue fabric panels that descended from the front of the waist.
This robe was paired with the Lorrdian Cowl of Quiescence, which shared the gown's color scheme. A Lorrdian male known as the High Priest donned this ensemble as he guided a Brotherhood of the Beatific Countenance pilgrimage to Jedha City on the sacred moon Jedha. This event occurred shortly before the city's destruction by the Death Star superweapon in 1 BBY. The Brotherhood's members, as part of their religious beliefs, obscured their individual identities, with others wearing orange robes. This made the blue cowl and gown a distinguishing feature of the High Priest.
The Blue Gown of the Sullen Moon made its debut in the Anthology Series movie Rogue One: A Star Wars Story, which premiered on December 16, 2016. Before the film was released to the public, the gown was presented as part of a promotional reel for Rogue One that was showcased at Celebration Europe on July 15, 2016.
Subsequently, the garment's title was revealed in the companion reference book Star Wars: Rogue One: The Ultimate Visual Guide, authored by Pablo Hidalgo and launched concurrently with the film. The name of the vestment pays homage to the 1989 song "Nightgown of the Sullen Moon" by the alternative rock group They Might Be Giants.