We're talking about good ol' nitrogen narcosis, yes, but in this case it's characterized by feelings of anxiety, doom & gloom, and/or a sense of imminent disaster, rather than the stereotypical description of narcosis as producing feelings of euphoria or intoxication. This is according to the accounts I've read - I've not experienced dark narc myself, and as far as I can recall had no idea the phenomenon even existed until many years after my initial training. One might suspect that this is one of those things that tends to get elided in the average OW training so as not to freak out the new diver...