(n or adj) A process that cannot be observed. This is commonly used in design procedurals, where the designer does not need to know /how/ a particular piece works, merely what its input and output requirements are. It has become part of common speech, addressing any situation where the observer cannot observe the subtleties of some subset of an observed process, be this technological, interpersonal, etc.
 (n) hardened data storage and/or sensors, commonly used to reconstruct situations involved in destruction of the device the black box is incorporated into. Although they are traditionally called 'black' boxes, they are usually highly reflective over a large range of spectra and are capable of acting as a transponder and/or beacon in many cases.
 (n) A nanotech device designed to completely disassociate whatever matter it encapsulates. This may be from an explicit action or from sensed environmental conditions, or even from inappropriate attempts to access the containment space of the black box. Commonly used in shipping sensitive materials from one location to another to prevent third-party access.
 (n) Popular piece of jewelry. It consists of a small black box made of plaited quantum-computing mesh, covered with diamondoid. Inside the box is a single nanotech memory storage element housing a single bit. During construction the bit is set to a random state based on radioactive decay; the computing mesh will detect any outside observation trying to reach in and will then self-destruct, taking the bit with it. The idea is to carry around a single bit of information not even the greatest archai can know.
When scanning is attempted, a black box will generally disintegrate in a quiet but elegant "poof!" then change colour to (say) transparent, although there are many options. The real decorative boxes are of course not intended to be opened at all, but the fake ones will open when given a complex cryptographic key that fits a cryptographic lock inside the quantum computer weave.
Recently fake black boxes have appeared intended for information smuggling. Many believe the black box fashion was started as a Cyberian memetic ploy to enable the smuggling.
The worst quality fakes are easily revealed by outside scans, but better kinds are rather hard to tell apart, especially without destroying them. Thats why you should buy your black boxes at Blax Black Block - we have the highest quality zero knowledge devices in the archiplex!