Whenever I have encountered that charred unburnable blackness I have always taken it as a sign of poorly flushed product. I’ve had accidentally over dried flowers I’ve brought back to a more enjoyable state with jars and boveda packs, but I have never had it affect the burn as you describe.
Every time I have had to chop early without proper time to flush before harvest I have had that problem. I’m not trying to throw any hate here or anything but it’s such a recognized thing around my area that I have never heard another theory on this.
Those joints that are hard to get lit, burn kinda funny, ash doesn’t want to fall off on it’s own, you tap but it still clings on. The ash is really dark, maybe even fully black, and you can have it go out on you but it wont relight without the end getting knocked off, which takes several crunchy taps against the ash tray. I’ve seen pounds returned to growers over that shit, and they hang their head and admit they didn’t flush it.
Conversely, I have never known anybody personally who ever bought a large quantity without burning one first and tapping that ash into the palm of their hand, examining the colour and consistency of the ash. Fluffy, as close to white as possible, never crunchy were always desired traits.
But I have never heard this discussed outside the question of flushing before harvest, so I’m intrigued by your experience with this.