So, one of the things across the Cosmere that we see is that, while the Spiritual Realm is something like a font of Investiture and a confluence of many, many things from the other planes, Investiture can manifest pretty much any way the shard commanding that world and dominating it sees fit. Preservation is REALLY good at this, working with the Mists, Snapping down to the level of being able to CHOOSE who and what abilities are granted, and, arguably, so is Cultivation. Cultivation's CHILD, not even Cultivation herself, can twist large elements of reality for individuals, and the queen proper has shown that she can grant the ability to convert matter directly to Investiture via lift, an ability we see extensively on Scadrial. Here's the thing. Nightblood isn't just ravenously consuming and developing themself, the way a human can with Breath, or, theoretically, a spren, given that they seem like they might be able to derive Investiture and Identity in part from the thoughts of creatures in the Material Realm. He's LEAKING. And he leaks the same black smoke that people puff into, just more concentrated, and that is canonically Breath. I'd be willing to bet he converts the Material and potentially Cognitive elements of what he destroys into Investiture and eats it, but it's super unclear how the Spiritual in general works.
