The RED/BLACK concept refers to the careful segregation in cryptographic systems of signals that contain sensitive or classified plaintext information (RED signals) from those that carry encrypted information, or ciphertext (BLACK signals).
Usage
In NSA jargon, encryption devices are often called blackers, because they convert RED signals to BLACK.
TEMPEST standards spelled out in NSTISSAM TEMPEST 2-95 specify shielding or a minimum physical distance between wires or equipment carrying or processing RED and BLACK signals. There are no requirements for the separation of RED and BLACK fiber optic cable.
RED/BLACK terminology is also applied to keys. BLACK keys have themselves been encrypted with a key encryption key (KEK) and are therefore benign. RED key is not encrypted and must be treated as highly sensitive material.