Flusser used the phrase “black box” to describe at least two very different objects: a camera (in Towards a Philosophy of Photography) and the postal system (in “Letters,” Does Writing Have a Future?). As radically different as the two seem to be, both refer to systems of human communication that contain some fixed segment that that is both necessary in order to communicate and inaccessible to the rest of the system. This is exactly its meaning in the context of science and engineering, where it was coined. It refers to a fixed, inaccessible segment or function within a larger, longer process or program. The function in question may occur inside something like an actual, physical box — which, coincidentally, a camera often is. But the meaning is metaphorical: The box encloses some activity that impinges on the rest of the system at two points only: input and output.
The “black box” on an aircraft, for example, systematically records events in the course of a flight as they occur in time, whether these are mechanical or human generated. That’s the input. The output is data that synchronises human activity, speech, and other sound with the shifting numerical data from the airplane.
To call a camera a black box, then, is to point out that between what goes in and what comes out, between all the concern with positioning and adjusting settings on the one hand, and photographs on the other, is “bracketed out” of almost everyone’s thinking about photography. A practitioner who manipulates what goes on inside – Walead Beshty, for example — is arguably skirting the edges of “photography” as such (something Flusser would have supported and encouraged).
The term does position the one who uses it, however, like a shifter in a language system (“I,” “you” “here” “there,” etc.). In the familiar situation of sending a letter or package to someone, for example, the postal service is a black box: to the sender and receiver, the service matters only at two points, drop off and pick up. But clearly it is not a black box for employees, managers, investors, etc…