A zone of space where the speed of light is reduced. By lowering the speed of light below the 3rd cosmic velocity, an entire planetary system can be hidden from the outside universe. Nothing would be able to leave such a system, making it essentially behave like a black hole. A civilization living within such a region of space would be harmless and no threat to others. That’s why a Black Domain is also considered a Cosmic safety notice.
Other civilizations could simply ignore such a species who decided to shroud itself from the Universe as it would mean no threat.
A civilization hiding in a Black Domain would severely limit its capabilities to develop advanced technology. Things like computers or advanced propulsion systems would not work in a world where the speed of light is only a few kilometers per second.