[Heroka Vendile]

yeah yeah guys, i know, i know, lol.

As for the black carapace, it would possibly exist in some form or other, but generally by the time a champion has reached the level of Daemonprince, I would expect their armour to be a permanent part of their body anyway, so if you breeched their chest armour, you'd probably just find mutated flesh welded to the inside.

EDIT:
Just as a wee extra, it should be noted that its not just CSMs that can become Daemonprinces... they existed before the Imperium for certain. And I'm pretty sure it would be possible for an alien to ascend as well. It takes a lot of bloody shed and centuries - if not millennia - of servitude to be given such a gift by a chaos god, but with the gifts of chaos, it would not be impossible for any sentient creature to achieve this.