Here are some possibilities, depending on what aspect of darkness you want to emphasize:
If you want to focus on the absence of light:
* Shadow: This is a physical absence of light, often cast by something blocking it. It's a concrete thing you can see, even though it represents the abstract concept of darkness.
* Void: This suggests a complete absence of everything, including light. It's a strong image that can be used metaphorically to describe the abstract darkness.
If you want to focus on the feeling of darkness:
* Blackout: This is a concrete event where the lights go out. It's a temporary state of darkness that represents the abstract feeling of being lost or uncertain.
* Night: While night itself is a time of day, it's often associated with darkness. It can be a concrete noun that evokes the abstract feeling of being lost in the unknown.
Ultimately, the best concrete noun to describe abstract darkness will depend on the context and the specific meaning you want to convey.