But Hussie said Eridan went Grimdark

Did he? I can’t quite recall that. Even if he went ‘Grimdark’ however, it was still obviously a different state than Rose, by the mere fact he was not under the Influence of the Horrorterrors. He was Grimdark, alright, in the sense that he shifted the Narrative to a much darker tone of death and hopelessness, Grimdark is not a Homestuck-exclusive word. We’re talking about the canon, in-setting State of being Corrupted by Horrorterrors, rather than the Framing Device of Eridan’s Snapping and the start of Murderstuck. Come to think of it, does Jade actually say ‘Grimbark’ in-canon or was this a Fanon Term that got super popular?

Okay no, yeah, it’s definitely a Canon Term. My stance on Eridan’s Matter still stands, though, there’s a difference between “Grimdark” as a state of being corrupted by the Horrorterrors, and “Grimdark” as a setting or a character being very… Well. Grim and Dark.

Not to say, Jade and Rose both kept on being ‘Grimdark’ until they died, whereas Eridan stops being framed as dark grey/black post-snapping, in the 3x Showdown Combo and before Kanaya splits him in half.