IX3244-G, also known as Fentakka, existed as a moon orbiting the gas giant planet Endor. The moon's surface absorbed the majority of light, reflecting under 10 percent, which resulted in it being the darkest moon among all of Endor's satellites.
Sources
Star Wars: The Rise of Skywalker: The Visual Dictionary (Initial appearance)
Kef Bir in the Databank (backup link) (Only a tangential reference)