Well, see for yourself in the screenshot. It looks bizarre. If this terrain is meant to look like it's floating... it just doesn't.
http://steamcommunity.com/sharedfiles/filedetails/?id=912547668
I have posted the (incomplete) map to the workshop for easy bug testing.
I thought the black was deliberate, to give the arcane chasm the impression of floating. IIRC, isn't the campaign's arcane fortress the same way?
On a side note, are the arcane pillars supposed to be purple? On both the theme and the terrain, the corner pillars have a purple tint to them. It looks a bit weird considering that the arcane theme's stone is otherwise imperial white, with purple for the non-stone items like cloths on the bookshelves or carpet on the floor.
In fact, this purple tint continues into the black dirt-less chasm beneath the pillars of the arcane chasm terrain.
My mistake then, I was looking at the wall itself rather than the chasm below. Sorry!
I am afraid you are incorrect regarding the cause.
In editor.
In game, with purple arcane theme.
This is in fact based on the chasm itself, with a weird colored texture wrap for the 'invisible' chasm beneath it.
I think it only looks weird because you're not using the arcane theme for the walls above it. The arcane chasm is effectively just a pretty transition between arcane walls and the chasm. The weirdness you're looking at seem to be some fortification on the walls which would extend down into the chasm. Either way it's intentional that it looks like that, it just doesn't blend very well with other tilesets.
If I recall correctly, the pillars (and the black directly beneath them) are always purple-tinted. You can see this in the screenshot without the arcane theme.