Blackstone Isle
Blackstone Isle is an island found off to the South of The Marrows covered in basalt spires. It has three locations upon it, a storage, the Ruined Mansion, and the Workshop. According to the Mayor "It technically falls under the jurisdiction of Little Marrow. [...] It was sold to a private owner some twenty years ago. [...] I've not met the owner; I suspect they don't live around here. The island seems quite uninhabited." The Manifest Ability can bring the player to the Blackstone Isle dock at any time. The Aurous Anchor also creates a portal to this island. Additionally, there is an Obelisk on the side of the island.
Ruined Mansion
The Ruined Mansion is a key location for the main story of DREDGE. If you try to interact with the Ruined Mansion before starting the main quest, a message will read "The door is locked." and the lights on the top floor of the mansion will be off.
The Ruined Mansion unlocks during the Pursuit "Grotesque Fish". After the Fisherman gives the Fishmonger an aberrant fish and is given the first Relic, a mysterious man invites the Fisherman to meet him at the Ruined Mansion on Blackstone Isle to discuss something. He is then revealed to be the Collector.
The Workshop
The Workshop is described as "Looks as though it hasn't been used in decades." When the Fisherman tries to enter, they are met with the simple message "It's locked." The Workshop can be unlocked by purchasing the Blackstone Key DLC. It has a dimly lit interior, and it is full of "Some horrifying creations amongst the detritus." being the Arterial Engine and the Sign of Ruin. If the option "Pick Up Items" is chosen a second time, the dialog reads "You've already taken the equipment that was stored here."