Is night VMC still VMC if the underlying terrain is completely unlighted?
The short answer is no, the definition of night VMC does not specifically require any defined degree of ground illumination to be present.  (at least in the US)

You raise some excellent concerns in your question, demonstrating a solid understanding of the principles of Operational Risk Management:  Identify the hazards, assess the risk, make decisions, implement controls, and monitor.

Following these steps prior to, and during night flight is crucial to safety for the reasons you describe.  As long as visibility is relatively clear and there is enough ground lighting to form a discernable horizon, flying on even the darkest nights can be done safely, and can actually be fun.  It is easier to pick out other aircraft visually, and roads and cities really highlight themselves.  In areas where illumination is sparse, situational awareness of your position and the highest terrain in your sector is crucial.

It is rare for anybody to truly need to fly VFR at night, so if weather, visibility, ground lighting, or any other conditions are concerning to you, either don’t fly, or file IFR if you are so rated.