Actually, that's not logical at all. Based on that logic, the following statement would be true:
"While I have never observed a lightbulb emit darkness, it is possible that a lightbulb could emit darkness, therefore darkness-emitting lightbulbs are possible."
That's the logic response.
The physics response is this: plasma is an excited state of matter. Matter, when excited, frequently emits light in the form of photons. This is the case with plasma - electrons in the atoms in question jump from orbit to orbit, releasing energy as visible light. From a physics standpoint, darkness is the absence of light, i.e., the absence of photons. Plasma cannot emit the absence of photons; for that matter, nothing can emit an absence of photons, therefore nothing can emit darkness.
Don't even bother saying "black holes." The keyword is "emission."