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: the Hito Hito no Mi, Model: Zaza—the Rain God Fruit.
What the world has long interpreted as pure darkness is in truth the nightmarish manifestation of underwater currents, irresistable and inescapable.

The first reaction I expect is, that you assume the fruit belongs 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). The assumption is rain god -> some power with rain. But the sun god nika has no power related to the sun. The sun is a metaphorical connection. It represents an idea, rather than a power. The dawn, the endless possibilities of a new day, the vast freedom of the open sky. I believe the same is true for the rain god fruit. It represents despair, coldness, depression, tears.

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. It selected its user.
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 Zoan have a will of their own. The Sun God fruit waited for the right user. The fruit of the rain god did 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” effect. It is the same fundamental principle that makes the ocean itself the natural enemy of all devil fruit users. Seastone 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 plunged into water.

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’s nightmare form is a moving body of water. Contact with Blackbeard is contact with that body. The “darkness” is simply how the human mind interprets the lightless, crushing depths. Over centuries, the myth of a man 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.

Davy Jones’ Locker has long been folklore for the bottom of the sea—the place where drowned pirates and their treasures settle. Blackbeard lives this metaphor. He collects. 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, discarded souls—exactly the kind of company one expects to find in the locker at the bottom of the world.
The fruit itself seems to facilitate this collecting. Just as the ocean floor gathers the dead and their belongings, the user of Zaza gathers Devil Fruit powers and broken men. 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 as the dark counterpart to Luffy. Nika is the Sun God—liberation, laughter, the burning will that refuses to fall or sink but 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 collects at the bottom of the sea. The celestial dragons fear both the D. 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.

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: the will to pull everything into the dark and keep it there.