Hi! I'm not sure if it was mentioned somewhere, but it bothers me a bit...
We know how did Nightmare lost his eye. But... In the comic there wasn't any goop or anything trying to cover him, so why did he turned black? Also, his smaller form doesn't have this injury. Well, this might be explained with something magic-related, that just his magic can't make whole big body anymore, but this still won't explain the goop(or whatever it is, cuz I'm not even sure about that XD)
Of course, you don't need to answer if you don't want to! I'm just curious, if there was something about it, and I missed it... Or if it just isn't related to Nightmare's trauma...?
That's okay! <3 It's a lovely question :D
Nightmare is an octopus so the goop/ink is his camouflage like how octopi camouflage themselves to fit in with their surroundings or ink to get away from predators. The goop is pretty much both of those combined into one to help Nightmare hide in the shadows. The blue is also because he's an octopus, since octopi have blue blood. Nightmare is basically always covered in a form of ink and his magic is the color of his blood because his body/mind is always in fight or flight mode to try and keep him safe (even when there's no threat).
The goop/ink was a slow process as he recovered, and Nightmare kept trying to force the shift which only made the situation worse. He strained himself too much.
For leviathans they can be injured, but they heal from pretty much anything. It's mental trauma that made him lose his eye and for his bigger form to be in constant camouflage form because it's how his body protects him. He's only injured in that form because that's the form he got injured in and he didn't allow himself to properly heal.
