The dark star skin line is inspired by black holes and destruction.
Orianna has an ability that pulls all opponents in it's vicinity towards one point (her orb).
So why not replace her orb with a black hole and give her a dark star skin ?
I tried a few ideas for her orb design with more or less "solid parts", but a black hole similar to the one from Interstellar should work just fine.
Also: Here's a link to the LoL Forum thread, where you can support the skin.