For night VFR you can either navigate visually or by navaids you're rated on.
If you choose navaids and have the equipment / training to use them(which may include an approved GPS) you need positive fixes as per the AIP IFR requirements for navigation.
Otherwise you're back to the VFR position fixing requirements, ie a positive fix every 30 minutes by identifying something on the ground. You said you have no reference to the ground, but that's not correct - if the weather's clear, as it should be if you're punching off on a NVFR nav, you will be able to use DR techniques combined with identification of ground lights to get positive fixes.
If it's a DR nav to the back of nowhere on a really dark night without the possibility of identifying townships or whatever on the way, you should probably rethink your route!
On a nice moonlit night you'll be able to see clouds, horizon, ground features etc and have that nice feeling of knowing where you are, but of course that's not going to happen all the time.
NVFR will always be fraught on a super black night because you won't know you're about to go into cloud until you're in it, but that's where a (hopefully) good forecast and go / no go decision come into it.
It comes back to the whole idea of NVFR - as I understand it, it exists in the regs to allow you to have a bit more flex regarding getting somewhere after last light, or launching before first light. In reality though, there's no restrictions on moon state or anything like that, so nothing to preclude you launching off into complete darkness with all the issues that come with that (illusions, possibility of CFIT, possibility of inadvertent IMC, etc etc).