Darklight

(Usually just referred to as darkness in-world, darklight is used in pedantic situations to make it clear that we're talking about magically-enforced, slow-moving negative light, and not just an absence of light.)
Strange substance that behaves somewhat similarly to a gas. Tends to spread at around ~50m/s undisturbed. Absolute pitch black and appears to actively drink in light. Muffles sound depending on thickness.
Those from the plane of darkness have a sense that perceives the world through this substance. Exact interpretations vary, but humans from the pitch black forest have darklight perception layered into their vision. Typically, darklight perception resembles echolocation more than it does sight in terms of mechanism. CoTD and other humans able to sense through it generally have adapted eyes which naturally 'layer' it into their vision, perceiving it visually. The view is often described as reminiscent of seeing double, although the clarity of view tends to be inversely proportional between the two (darklight blocks light, so a clear darklight 'image' would not be perceptible on the visual spectrum).
Darklight can be ambiently created in pitch black rooms of certain regions of the world. Intensity varies depending on the area's connection to darkness. The central lands tend to have very little, the equivalent of a very weak flashlight. On the other hand, the connection in the North is strong enough to have darkness straight up form a connection to its plane and leak darklight through.
Shadowspeak is a language 'spoken' through the medium of darklight.

Physics

It is very important to note that darklight is not simply inversely proportional to the amount of ambient light. Darklight exists as a magically-based state of 'negativity' on the visible (and often IR) spectrum infused into a gaseous substance. Ordinary darkness is simply the absence of light, while darklight actually goes below 'nothing'. It doesn't necessarily annihilate on contact with standard light so much as overlapping- perceived pitch black is simply where the negative value of the darklight exceeds mundane light. Cotd eyes are magically altered to perceive light levels below zero as a sort of inverse light, allowing them to perceive the world through darklight, and separate the magical and mundane components of light (allowing them to ignore darklit in lit areas). As a result, in a pitch black room within a region which somehow has zero link to Darkness, Cotd would be blind as anyone else. Their 'night vision' is a consequence of Darkness being a greater plane, always invoked in circumstances related to its aspects.
There are a few edge-case interactions between light and darklight which arise as a result of this. Darklight overlays an overwhelmingly high value over the visible wavelength band, effectively blotting out light. Ghost Caps are an odd case, essentially a naturally occurring illusion.
Despite being coloquially referred to as a substance, darklight isn't strictly one. It has no mass, for one (Umbrite gains mass due to air, or whatever other ambient medium exists, compressed into the material, an intrinsic part of the stabilization process). Some argue that darklight consists of two components- the 'negative photons' themselves (a blue quanta effect), and a gaseous substance that they are suspended in.
Another, more technically correct, way of defining darklight is as a blue-quanta phenomenon that permeates, or possibly conducts, through matter at a relatively slow speed. The fact that it occludes sight is only a part of this quanta manifestation's function. When one sees swirling darkness, it's darklight attached to the air drifting according to air currents.