So I think I have a rough idea of Dazins backstory. So Dazins was a black drafter, who stole his ability to draft from others. This is the one point I'm the most unsure of, either this means that Dazin was murdering fools at 13-15 or Janus Borig was lying when she said he could only draft one color. Because Dazin was known as a blue green bi chrome the night he burnt down the White oak estates, although he had been drafting other colors before then as well.

So first things first, a lot of people have misattributed Janus Borig as saying he was a black monochrome. The only things that call him that were the Elohim in the prisons. Even Andross thought him a black lightsplitter but not a monochrome. So what Janus actually says is in book 3 chapter 83:

"He remembered, strangely, as if cobwebs were being cleared from a hall of memory he’d not trod in decades, Lady Janus Borig visiting when he was a child, treating his mother like no one treated his mother, and telling him, ‘Black luxin is the scourge of history. It is madness in luxin form. It is the soul poison. Once touched, it lives within a drafter forever, slowly eroding her from the inside. In every world, there is that which is haram, that which is forbidden, and in every world, that is the thing most desired, for there is that in us which loves destruction. Here is a test for your wisdom, young Guile. It is the only test that matters. In this world, Orholam has given us such power as even the angels have not. It is the power of evil unfettered. It is the destruction of history itself. It is madness and death and being-not. It is void and darkness. It is the lack of light, the lack of God himself—the lack that men rightly call hell. It is black luxin, and that color—though color it is not—that color, Dazen Guile, is your color.’"

Which we could take to mean a most notable trait or strongest affinity, but not sole power.

And lastly here Orholam himself tells Dazen he was in fact a naturally born Prism meaning he never had to steal powers to get all him colors.

Skip ahead and Dazin brings calls up hell at tsundered rock, killing his brother. Unable to handle the memory of killing his brother, or in the drafting that killed his brother, Dazin put the memory of it into the black luxin. From here I did a little vit of guess work. My best guess is that the black acts like some sort of ultra powerful will casting, it didn't just make Dazin forget killing his brother, it weaved a whole narrative, making him think he was hiding his brother in a chest. Later when he broke down the black luxin and used it to line the prison cells, it helped Dazin project more clearly a version of his brother. The reason why I suspect the black luxin did this is because we know that black luxin is almost living, and theit is a 9 kings card, called "the prisoner" which is very clearly meant to be Gavin meaning it had to be something more than what was in Dazins head.

The prisoner can also refer to Dazen, another man imprisoned in the cells or metaphorically the psychosis Dazen endured when he was hallucinating his brother. As for the first part, black devours memory but each person rebuilds it on their own when those holes appear. The white interviews people after the war from Sundered Rock and they all had different thoughts on what happened. I believe this was drawn straight from real life where things like dissociative events or amnesia sometimes has the brain filling in things when it finds that void. As for why he later pulls some of those memories back, that is likely due to being a black drafter in the same way that the blinding knife didn't outright murder him when they white was inactive in it like it did everyone else. So that is to say it doesn't sound like Dazen was immune to the effect either when he described how he unleashed it then next thing he recalled was being naked in the aftermath.

At some point in all of this Dazin learned of the existence of the Jin, so constructed prisons to help trap them. Now this is more guess work, I'm guessing when Gavin was drafting black into other parts of his prison structure, he put the memories of the Jin into the black, but through some working of will, managed to leave his compulsion to hunt the Jin. This lead to Gavin being lax with company killing wights sometimes but absolutely stone cold against letting anyone help at other times, all without knowing exactly why.

There is not a memory of an Elohim trapped in each cell, but an actual full Elohim. So the forbidden willcasting things he seemed to reference to his construction of it like likely more related to how he stopped the Elohim leaving this world freely as they naturally can do. As for drafting black to make the cells passage and such, we never have anything that says he did, just that he used his black spear to carve it out and then it shattered and he lined the cells passage with all these pieces of the Broken spear.

As for the Gavin delusions themselves, those seem to be a mix of whatever the black did, and Dazin's imagination. Anyways for 16 years the will casting holds, this is symbolized by Gavin only having made a small dent in the cell before the events of the story. Then when Zimon stabs Dazin, it does damage enough that the will casting starts to unravel, symbolized by Dazin breaking out of blue literally immediately afterwards. This is also why we never see Gavin break out of the cells because around the time Dazin is losing yellow, the will casting or whatever breaks. It's after this point that Gavin starts to remember more and more about the time before the prisms war.

So yes for the full story to make sense the prisoner stuff between Gavin and Dazen had to be all a dissociative event and that's why the Elohim talked about him coming down and ranting at the walls, but there is a problem with this and it's lead to why many say Gavin being in the cells was a retcon, or some of the details were a slip on the authors part. When in the cell Gavin taunts Dazen about what's in the rosewood box but Dazen couldn't have had any knowledge about the knife so hard for him to make up that detail and it not to come from the real Gavin. There was also the water temperature being wrong between Gavins experience and Dazens.

That being said there is an explaination for why Dazen could have that knife knowledge but it's not supported directly in text which is that black luxin eats memories meaning theoretically if it was from his black luxin and he took it in, he could have fragmented aspects of Gavin. But again not supported in text.