Grimdark is among the things out there with a whole lot of speculation and not a lot of examples to base that off of. As far as in-canon explanations go, we get "the fabled blackdeath trance of the woegothics" and "completely off the deep end in every way" - neither of which are very specific, but we work with what we got. How you regard that also entirely depends on whether or not you regard Grimbark to be worthily considered, as well as whether or not you believe Eridan went grimdark during Murderstuck.

As far as Rose and Jade go, they're thankfully described pretty consistently. We see Rose "go off the deep end" and fixate on the one goal of revenge; whereas Jade is less entirely/strictly just "under the control" of the HIC so much as she's having her "inhibitions" let loose. On top of that, Rose has pretty clear, direct connections towards the Horrorterrors that lend a way for her to become influenced by then; whereas Jade is being double-mind-controlled by the Condesce, whose lusus - Gl'bgolyb - was confirmed in a book commentary to have been an actual horrorterror brought to Alternia by Doc Scratch. While less direct, you could still use that to argue that a connection to Horrorterrors existed, given HIC has contact with and shares a mental connection with said horrorterror.

Based on that, I'd personally point to those few factors, albeit still pretty vaguely.

1. The transformation you undergo is that of eldritch influence.

2. It marks a loss of control or inhibitions, specifically in a bleak sense (as to contrast the loss of these seen in its inverse of Trickster Mode, and hence its spoken tongue being referred to as "broodfester")

I've also seen a handful of talk out there about how it relates to your aspect, and how it can be indicative of inversion, or over-embracing it, or undervalue it, etc. but I personally struggle to find worthy evidence of this. While it's true that Rose going Grimdark is especially "Voidy" - I think it's better interpreted, or at least more narrative satisfying, from a personal perspective than one of game mechanics. Rather than Grimdark being what happens when you are "heavily resistant to their trUe calling" or "corrUpted in some way by an oUtside influence" - I prefer to see Grimdark as specifically something that occurred when Rose Lalonde rejected her calling as a Hero of Light - as Grimdark with all it's voidiness, obfuscation, and what have you is what would be Rose's way of manifesting powers in defiance to her aspect. What you define to be "in defiance with your aspect" should be caught under a much broader umbrella than Grimdark, and up to each person to decide best fits their OC and how they engage with their role.

I think the most important takeaway you can have though isn't from canon at all. In the end, we know JACK SHIT, but Grimdark still sits there as a very powerful narrative tool just waiting for its potential to be sapped. If you end up having an idea for what you want to do, and you worry that it may not align well with some invisible, unspecified guideline on how Grimdark works, remember that telling a good story will always be more satisfying in the end than "being accurate." I think a lot about how Grimdark works because I find it fun and interesting, but when it comes to OCs or AUs, I sort of go "to hell with it" because you'll get the most out of making Grimdarkness something PERSONAL to your OC as far as why they're driven to such a dark and volatile point, what it means to them, and what you intend to accomplish with it.