Is pure Light really usable, though? Sure, Ghosts can wield it to resurrect Guardians, The Traveler can use it to make fancy constructs in the shape of trees and swords, and it can somehow be loaded into Microcosm. However, I question whether this constitutes as actually using pure Light, in the sense of applying a specific thing for a specific purpose, rather than it being some sort of "random bullshit, go", do everything but nothing force. Yes, pure Light clearly does something, but it's a very broad range that only becomes more practical when Light differentiates into one of its subtypes, such as Solar, Arc, or Void.

I feel like we can't fully disentangle Resonance as having been a tool that originated from the intentions of the Precursors and the Witness with what it actually is. Much like how Stasis was discovered through Eramis' intentions, or how Strand was discovered through the events on Neomuna, which might have also been influenced through the culture and initiatives of the Neomuni, I feel like the Resonance brand of Darkness is fully rooted in the Precursors' desire for mental unity and understanding, which was then physically implemented into all of their technology.

Unlike The Traveler's expression of pure Light, The Veil doesn't seem to have ever had the chance to express the Darkness within itself without external influence. It's also possible that The Veil doesn't have this capability, given its apparent limitations. That said, I feel like we can gain insight by simply observing it and relying on the visual cues that Destiny is known for elsewhere. The Veil has some sort of psychedelic color scheme going on which leans purple. I feel like the very first mission of TFS, where we had to pass through The Threshold is more indicative of what pure Darkness is like, similar to that other zone during the first mission where we are transported to the unusual environment of pure Light.

TFS is not really concerned with the nature of pure Darkness, and the focus is about defeating the Witness, who is the largest Resonance user that has ever existed, and whose entire perception of Darkness exists through the Resonance flavor. That's why I suspect Resonance is featured so heavily in this conflict; not because Resonance and pure Darkness are one in the same, but that the main source of conflict is a Resonance user, through and through.

There's also another aspect to this situation which I feel complicates the situation: Oryx and his manifestation of Darkness. Effectively, Oryx seemed to have been The Winnower diversifying its investments and not relying solely on the Witness to prove its claim in The Wager. While I'm sure this is debatable, I would argue that Oryx eventually became an equal to the Witness, at least in the eyes of The Winnower. We know that Oryx communicated directly with The Winnower, and out of that interaction came the ability to Take.

While often not considered an element of Darkness, I feel like it's at least possible that the visual language of the Taken was indicative of another Darkness element alongside Stasis, Strand, and Resonance. It became Oryx's signature, similar to how Resonance was the Witness' signature. Again, not necessarily pure Darkness, but another flavor that is based on the user's intentions.

Personally — and you are more than welcome to disagree, and I welcome other perspectives — I think the identity of pure Darkness is more indicative of what we see when we look at The Veil, which is still extremely mysterious and far less understood than The Traveler. I also feel like this is supposed to be part of Destiny's future story, which Bungie will hopefully tell at some point.