The Exarch's diorama represents the 7th Calamity with the Seventh turning this weird "evil dragonfruit" color that we sometimes see when Darkness is involved (more commonly we see that evil grape color mixed with black, but we see the evil dragonfruit color, too).

The lore book told us already that the 7th Umbral Calamity caused by Bahamut just before ARR was "all elements charged astrally". If you take this in conjunction with Urianger's revelations (that the presence of Darkness will give elemental energies an astaral charge, and vice versa, and that Eorzea has incorrectly assumed the opposite for millennia), it all fits, and thus reveals that the 8th Umbral Calamity would have been all elements charged umbrally by the rapid increase in Light on the First (which was already Light-heavy but in delicate homeostasis).

The big takeaway here is that, yes, it's convoluted, but lore integrity counts for a lot and there weren't a lot of ways to keep it all consistent.

Especially since Calamities are elemental ... astral and umbral are elemental polarities ... and Light and Darkness are not even on the elemental energy spectrum.