In the Universe of the Black Blood Saga, also commonly called the World, there are many worlds that fall into certain realms. All of these realms are on their own plane of existence. The Universe is similar, but in some senses different, to the Universe of canon Kingdom Hearts.
Before the Universe became what it is, the world was bathed in endless light. However, hidden in the light was greed. The people began to fight over the light, both for good and bad, and darkness consumed more hearts than one. In the end, the rulers of Dark and Light destroyed each other, and dropped the world into endless Darkness. Setting the world flowing into the path of destruction, as Kingdom Hearts itself sealed its door and went to sleep.
Seven Lights, the Seven Princesses of Heart, shined their divine light across the worlds and with the light within the hearts of children saved the world from melting away into the Darkness. But at a cost, no longer was the world one single union, now each world within the larger whole was divided and separate.
However the events of the past stretch on into the future, Darkness and Light would forever strive against each other in an uneasy balance, only to re-enact the massive war from the dawn of history again and again once the tension reached its peak. The events and outcome within the war, would decide the future of the worlds for the next cycle of balance. This, was the new world that Kingdom Hearts gifted, with its silence.
Realm of Light
The Realm of Light (光の世界, Hikari no Sekai?) is a realm made up mostly of light and makes up almost half of the Kingdom Hearts' canon universe, similar to the Realm of Darkness. Like the Realm of Darkness, the Realm of Light has its own Keyblade group, the Keyblades of Light.
Realm of Darkness
The Realm of Darkness (闇の世界, Yami no Sekai?) is a realm made up of darkness and makes up almost half of the Kingdom Hearts' canon universe, its counterpart being the Realm of Light. Like the Realm of Light, the Realm of Darkness also has a Keyblade Group, the Keyblades of the Dark. Little is known about this realm, save for the fact that it is the home of the Heartless, and Kingdom Hearts is found within its deepest depths.
The Door to Darkness
The main entrance to the Dark Realm, is called the Door to Darkness. The Door to Darkness was actually revealed to exist by the power of the Dark Emperor, he used Fallen Deity to create the first Door to Darkness during an important battle, a portal into the deepest Darkness which he used to summon an infinite army of Heartless and spread them out into the Worlds.
Only one Door to Darkness can exist at any one time,
The Realm Between is the third realm in the Kingdom Hearts series, and lies on the boundary between the Realms of Light and Darkness.
Few worlds exist within it, and most have some special nature or purpose. Each world ties closer to the Realm of Light or the Realm of Darkness, with the exception of Twilight Town, which exists exactly in the middle. However Shedo Dark states that most of the worlds of the Realm Between have Hearts made, or remade, purely of Darkness.
The Realm Between is also known as the Realm of Twilight, it lengthens as light declines, therefore Dark Keyblade Masters are able to corrupt the hearts of worlds on the boarder from Light to Darkness to swell the Realm Between's ranks. The Realm Between, instead of the largely incorporeal Realm of Darkness, is the home of the Dark Empire and other dark powers.
Worlds in the Realm Between, specifically those corrupted, are immune to the threat posed by the Heartless. Their hearts fully corrupted to Darkness, whether having become that way naturally or corrupted by an external influence, exudes no light to beacon the Heartless. Additionally, when a Dark Keyblade Master corrupts a world's heart, they also lock the Keyhole of that world's heart in the process thus sealing the only access there is to the World's Heart until another Keyblade unlocks it once again.