I was going to post a bug report on the tracker but thought I'd post here first as I'm unsure of the conditions which actually cause the issue.
The issue (and fix) is discussed here (specifically post #11 on the 2nd page) but the TL;DR is that it's where the game randomly sets the gamehour variable to a negative number and upon transition to a new cell the view is pitch black but your avatar can still move.
I'm unsure whether it's a bug that exists in vanilla FO4 or whether it's caused by a mod.
Assuming it is a vanilla issue, does anyone know if it's an issue with the engine or whether it can be fixed via the CK?