So let's focus on a few things here, Gavin(real), Gavin(dazen), seed crystals, Jinn and their influence, and probably a few more things. By the end of we may come to agree or disagree.
seed crystals, what we know is that they enhance powers over a colour, greater perception to the colour, they are produced by what seems to be resonance points of a specific colour throughout the world, they draw drafters, and more powerfully wights, to this source, and probably a host of other things i have forgotten. For the black seed crystal it has something to do with the prism ceremony, as it is guarded by the blackguard and the white. And the specific one we see has been in the chromeria for the entirety of Gavin's reign.
Two things on jinn.
1. what we know, they want to become one of the 9, that is their ultimate goal as far as we know. They follow around wights/or strong drafters for the chance that they become wights, find the seed crystal and combining become a God. They are only visible to the specific drafter at that point, they take on a form to earn trust.
First example is Samilah and Meena. Samilah always seems to talk down to her, Meena puts forward an idea and Samilah says always something similar to no your logic is flawed this is a better idea. So this jinn seems to feed her superiority over intelligence and logic, which if you wanted a blue to trust you would be the ideal way to do it.
Secondly Liv and Beliol. Liv wants to be worshipped, powerful, above all else. How then does a jinn earn her trust? No lies, hard truths and serving. Liv knows he is a jinn, she figured that out and that Samilah had one. So what feeds her ego more than having one of these beings serve and worship her.
2. How they influence drafters, i feel this is kind of like will casting for the jinn. When a person drafts they are allowing this luxin to pass through them, and through that the jinn can influence with their will, providing the metaphysical characteristics that we see in specific colour drafters.
So now the Gavins and the Nature of Black luxin
First Gavin(real), he was crazy, he didnt see will cast images in the luxin he saw his reflection and had conversations with himself.
Secondly Gavin(dazen) is not crazy, well not in the way we are falsely lead to believe. I believe he is a black wight(since the battle of sundered rock). Firstly he didnt will cast into the prisons, the dead man gavin sees is a black jinn. Secondly black doesnt make you forget but it definitely is the colour of madness. What makes a person more crazy than to undermine one thing that they have always relied on... Gavin's flawless memory. To put shadows of doubt over what he believed to be the truth making him question his sanity. He doesnt fall for it though, well not completely, because he is strong willed and what do we know about strong willed wights. The jinn doesnt have the same effect on them as a weak willed wight. They are less malleable, take a lot longer to gain trust and i have no doubt can cast out jinn as we saw gavin do in the black luxin cell.
So why is there a black jinn sitting under the chromeria? who knows but we do know that they have had a pretty stable supply of black seed crystals there. If jinn are in the luxin and resonance gives to seed crystals then that could explain their supply of these seed crystals for prisms every seven years.
Lastly just a bit of speculation, if all prisms are black drafters i think it has something to do with incorporating the black seed crystal into their bodies. This explains the limitless drafting of a prism as the black slowly eats away at the luxin that is stored up in their body. And when a prism "breaks the halo" it is just them being given over to the black and they go mad, which is how it is explained in the books. So the way koios became a black drafter is by incorporating the seed into his own body and he instead of killing people for his colours is planning to siphon them from the God's themselves, with the black luxin collars(as is hinted to by andross).
If you go back and read TBM as though he is a black wight, his interactions with the dead man seem to make more sense.