Mythical Hito Hito no Mi: Model Zaza

The central claim of this theory is that the Yami Yami no Mi is not a logia of darkness at all.
It is a Mythical Zoan: Hito Hito no Mi, Model: Zaza—the Rain God Fruit.
What the world has long interpreted as the devil fruit of "darkness" is in truth the manifestation of the deepest sea, the irresistable and inescapable darkness of the bottom of the sea.

Also known as Davy Jones' Locker.

The first reaction I expect is, that you assume that the fruit of Zaza should belong to Dragon, because we expect great things from him and he is already connected to weather phenomena, like heavy wind, rain and potentially thunder (logue town). Your assumption maybe "rain god" so there needs to be some connection or power associated with rain. But the Sun God Nika has no power related to the Sun either. The sun is instead a metaphorical connection. It represents an idea, rather than a power. The dawn is a symbol for the endless possibilities and vast freedom of a new day and the open sky. I believe the same is true for the rain god fruit. It represents despair, coldness, depression and tears.

So, Blackbeard?
During his clash with Ace on Banaro Island (Chapters 440 – 441), Blackbeard makes a the statement, that the fruit "chose him". After explaining how he had spent decades waiting for the fruit, he declares that the ability chose him. The fruit did not simply fall into his hands by chance. He claims it selected him.
In the context of Mythical Zoans, especially those tied to gods like Nika this language is crucial, as we learned from the Five Elders themselves, that all Zoan have a will of their own. The Sun God fruit waited for the right user while avoiding the World Government. Has the fruit of the rain god achieved the same?

The Yami Yami no mis most terrifying ability is its capacity to nullify other devil fruits upon contact. This is not an arbitrary “darkness” or "gravity" effect. It is the same fundamental principle that makes the ocean itself the natural enemy of all devil fruit users. Sea Stone is said to emit the same energy as the sea. Blackbeard’s touch produces an identical result: the sudden, crushing loss of power, as though the victim has been touched by Sea Stone. There might even be a connection to the way pyrobloin enters the atmosphere (vulcanos? Perhaps even underwater vulcanos..?)

If the fruit is Zaza, the Rain God, then this makes perfect sense. Rain is water falling from the sky; the ocean is water collecting on the floor of the world.

Killinghams imagination of Zaza, his own nightmare, has the form of a literal moving body of water. That particular design of Zaza could be the manifestation of rumors. Touching a user of the Yami Yami no Mi, robs you of your powers, as if you touched sea stone .. As if you touched a body of sea.
Over centuries, the myth of a godess whose body is darkness and touching them robs you of your powers, as if you fall into the sea, may simply have distorted into a cultural memory of a user whose body was the sea itself. A being made entirely of water.

The Davy Clan?
Davy Jones’ Locker has long been folklore for the bottom of the sea. The place where drowned pirates and their treasures settle forever. Blackbeard lives this metaphor. He collects, just as the sea floor does. He took Whitebeard’s Gura Gura no Mi from a dying body in the chaos of Marineford. He descended into Impel Down and specifically recruited from Level 6, the deepest level of the prison, the one closest to the sea floor in both literal and symbolic terms. His crew is a collection of the most dangerous souls. Discarded and forgotten and the bottom of the ocean.
The fruit itself seems to facilitate this collecting. Just as the ocean floor gathers the dead and their belongings, the user of Zaza gathers everything. It can absorb and take in everything.
The dual-fruit phenomenon, previously thought unique and unexplained, becomes a natural extension of a power that embodies the sea’s tendency to claim everything that falls into it.

This reading places Blackbeard once again as the dark counterpart to Luffy.
Nika is the Sun God - liberation, laughter, the burning will that refuses to fall or sink but instead remains high above in the sky.
Zaza is the Rain God—the relentless downpour, the flooding of the lower world, the force that drowns and gathers everything at the bottom of the sea. with an endless hunger.
The celestial dragons fear both the Davy Clan and Zaza for related reasons: both threaten the artificial height and dryness of their elevated paradise. Rain, to those who live above the clouds, is an alien and terrifying phenomenon. A man who can become the embodiment of that phenomenon is a walking catastrophe. A Davy, who has eaten the fruit of Zaza, may just be their downfall.

Blackbeard’s entire career reads as the methodical work of someone who understands that the sea does not hurry, but it always claims its due. The fruit recognized in him a kindred spirit:
A man who will pull everything into the dark and keep it there.