The Black Sea is a truly ghoulish Shard, appearing for the most part as an endless ocean of black, oil-like liquid with sparse, lifeless islands and beaches here and there, the sky always being full of black clouds with an endlessly full moon shining on it from above. Underneath the waves isn't much better, as due to the darkness, it's quite hard to see outside the presence of luminescence here and there, and the environments range from endless deserts of gore, twisting, tentacle-like kelp, jagged coral reefs, underwater lakes of blood, craggy stones, and volcanoes which just release toxic smoke and nothing else, while the abyss just seems to keep on going, separated into various blackish islands in a never ending descent of crags and darkness. The liquid of the Black Sea, known as Blackwater, is an eldritch substance that is the vessel for Nanami's endless curses, and as such has a corrupting presence, with intense protection being necessary to safely wade through it, let alone swim, without dying or worse, and those who die in the presence Blackwater, even if they are from other Shards, will resurrect as Drowned. Nanami lives in the ancient castle of the Seven Seas at the center of the Shard, which has no grown massive and seems to descend endlessly into the abyss as well. While it has the vibe of a cultured castle, its insides are quite disturbing and corrupt, and it's a host to the strongest and most dangerous of all the Shard's many evil denizens, as well as the center of its correspondence with the Tower, meaning there are always many Thaumaturges inside as well. Because Nanami has united the Shard in the corruption of the Black Sea, the Shard seems strangely calm most of the time despite the perpetual high winds and storms that are common all throughout the Shard, adding to the creepy environment, but encounters with the locals will always dispel any thoughts that it might end up being actually peaceful, as they are of course all completely hostile to outsiders. The Shard is endlessly wrapped in Curses, so much that when a Sha monolith was constructed in the Shard, only one Khanate could be created before it literally exploded. Similarly, Blessing-based beings will quickly die within the Shard simply due to the saturation of Curses, and even Curse-based beings may be overwhelmed by the Curses within, leading to the Sha's tendency to avoid it. The temperature of both the water and and the above ground portions is below freezing, adding to the need of protection within it - indeed, foreign water will even freeze over time inside the Shard.

The inhabitants of the Black Sea are the seven races that originally inhabited it - just corrupted and transformed. They are as follows: The Drowned, who took the place of humans, waterlogged, partially rotted zombies satured with the Black Sea's power, the corpses of the humans who rapidly died in the creation of the Black Sea. They are not limited to being human however - any foreign lifeforms that die in the presence of Blackwater will become Drowned. They have cruel personalities and unlike many undead retain full cognizance, just developing a deep sense of resentment and cruelty. The Abyssals, who took the place of the Sea People, being the same humanoids with aquatic traits, just influenced and corrupted to become more harsh, creepy, and dark in appearance, as well as gaining more creepy and otherworldly qualities - as well as being Nanami's personal court. The majority of settlements in the Black sea are Abyssal settlements. The Selkie, who took the place of Mermaids, gaining much more monstrous features and disturbing scales, as well as becoming feral and beast-like in personality, though still sapient - they just don't often act like it. The Undines, who took the place of the Naiads, becoming a pale-skinned race of female conquerors who reproduce extremely quickly and are constantly infighting with one another, and have an aesthetic reminiscent of Abyssals from Kancolle, and similarly creepy and detached personalities. The Harpies, who took the place of Sirens, gaining more messy and wild feather arrangements and more beast-like eyes, as well as becoming extremely gluttonous and obsessed with devouring other creatures, often gaining the abilities and aspects of creatures in the process. The Shapeshifters, who remained as they are in shape, but became darker in appearance, with razor-sharp teeth and overall cruel personalities, especially among the dolphin-types, who are violent, infanticidal, and creeps. Overall their personality is the most hostile of the beings within the Black Sea. Finally, the Dragons were replaced with the Kraken, tentacled humanoids who took their place and serve as the most powerful servants of Nanami, and all live within the endless Sea Castle, serving her whims and preparing the Black Sea's missions - as well as being their diplomats with the Tower.

The Black Sea has a demented sort of family-like vibe to it, with all the inhabitants loving each other and their company, even the Undines who just see death and infighting as normal, and are completely loyal to Nanami, and via Nanami, the Tower. On the other hand, like their masters, it is hostile to every single Shard not connected to the mysterious beings that run certain Shards like Merlynne or the Thorn, and have an especially earned reputation as one of the single most monstrous and brutal forces in the entire Network, especially given how easily they corrupt others to their side. The Shard Union rates them as an existential threat to many other Shards, though not all of them, due to how Nanami could theoretically infect other Shards' hearts with the Black Sea, and they are often reaving and razing other Shards, while attacks on the Shard itself are risky due to how all the fallen will join the ranks of the Drowned. They are completely loyal to the Tower and see them as great teachers, and have an especially good relation with the DD despite the Tower being neutral to it, since they are both cultures which glorify cruelty and negative emotions. Nanami is famous as one of the strongest rulers of any Shard, though she is weaker than the Tower monarchs for the most part, and the Santore Emperor. Because of the destructive potential of the Black Sea, it is considered a boogeyman among Shards, and stories relating to what sorts of sordid creatures and monstrosities one could find within it are common, often to the point that there are rumors of things within the Shard that are not actually there at all, like old tales of far off seas during the time of Pirates. The Shard treats Nanami's boyfriend like a devil figure, mocking him for being the only person who died during Nanami's awakening that she did not choose to resurrect.