I think it's a question of precise terminology. I would agree that it's a story about very dark, bleak subjects, and I think I'm coming from a place where - and this is probably a case where we can blame the media - "dark" is a term I typically see applied to something where there's more action and events. Because this fic was comparatively quiet, by which I mean more of the conflicts and actions came about because of the characters interacting rather than being acted upon by outside forces, it didn't feel very "loud" - so it hit me more as being a tragedy than a darkfic.

That it covered a long time frame had a lot to do with my seeing it as a tragedy, too - a gradual dimming of the lights instead of everything being snuffed out at once.

So we're talking about the same things and using slightly different words for them, I guess.