Solid Shadow

Energy projections formed from darklight. Unlike hard light, which is perfectly rigid, solid shadows are a flexible, lingering material. This shaped the way that Cotd and Abyss Dwellers' approach magic: hard light naturally predisposes one to creating static, unchanging objects, while solid shadows allow for more plastic deformation.
Darklight itself is a magical 'material' that exhibits properties somewhat similar to gas. Using magical energy, one can compress it enough to form solid objects. As a result, once the energy binding the darklight disappears, it dissipates and loses form. Umbrite is formed by utilizing this phenomenon, along with aligning the structure of the compressed darklight in a rigid, crystalline pattern.
As a simple example, consider the difference between the two basic offensive spells of each practice: hard light's Light Knives , and solid shadows' shadow whip. One produces extremely rigid projectiles, while the other relies on the flexibility of the construct to deal damage.
Solid shadow is only the first 'level' in Cotd sorcery. It is the only sorcery medium which ordinary humans can hope to master, but still requires extensive study and mental flexibility.

Dimensional implications

Hard light is commonly known to be the 3D shadow of Celestial manipulation.
Solid shadow is created by binding darklight into a coherent form. What does this mean, though? Put simply, it's a strange configuration of negative light quanta which results in the exertion of physical force. Energy is spent to hold the quanta in the relevant positions, not to project it (unlike hard light, which is entirely the caster's soul at work). One may expect a level of symmetry with hard light, it being some sort of higher-dimensional imprint of a 'lower' dimension, as a result of the association between demons and darkness. In reality, this isn't quite true. The substance is actually sustained entirely through a series of kinetic quanta interactions, the lack of Celestial quanta crucial to its flexibility. It's not at all a Chaotic substance, excluding cultural associations.

Some confusion stems from the fact that 'black body' is one of the several appearances that Chaos phenomena can take (the most common others being constantly shifting or straight up impossible coloration). While it may be visually similar to be solid shadow, it is something entirely different. See Realities:Chaos (Planar):Chaotic & Eldritch Images
Solid shadow and the umbrite infusion process bears some similarity to dilith, though unlike the aforementioned material, it does not have a distinct inactive/active state.