The term "black box" was originally a World War II British phrase – relating to radio, radar and electronic navigational aids included aboard British and Allied combat aircraft, back when those often-secret devices were black.

These days “black box” is the colloquial term for the flight recorders placed in an aircraft for the purpose of facilitating the investigation of aviation accidents and incidents, and these days they are mandated to be bright orange for reasons of visibility and recoverability.

There are two types of black box devices: a flight data recorder (FDR) that preserves the recent history of the flight via the recording of dozens of parameters (e.g. 88 by mandate in the US) collected several times per second, and a cockpit voice recorder (CVR) that preserves the recent history of the sounds in the cockpit, including the conversation of the pilots.

The two devices may be combined into a single unit. Either way, and together, the FDR and CVR objectively document the aircraft's flight history, which may assist in any later investigation.

They are built to withstand all manner of trauma. Modern FDRs are typically double wrapped in strong corrosion-resistant stainless steel or titanium, with high-temperature insulation inside. They are accompanied by an underwater locator beacon that emits an ultrasonic "ping" to aid in detection when submerged. These beacons operate for up to 30 days and are able to work while immersed to a depth of up to 6,000 metres (20,000 feet).

All black boxes are lettered "FLIGHT RECORDER. DO NOT OPEN.” on one side in English and “ENREGISTREUR DE VOL. NE PAS OUVRIR.” in French on the other side.

Both flight recorders are required by international regulation, overseen by the International Civil Aviation Organization, to be capable of surviving the conditions likely to be encountered in a severe aircraft accident. For this reason, they are typically specified to withstand an impact of 3,400 Gs ((1 G is the force of Earth's gravity, which determines how much something weighs) and temperatures of over 1,000°C (1,830°F).

(As an aside, US comedian George Carlin once posed this not-unreasonable question: “If the black box flight recorder is never damaged during a plane crash, why isn’t the whole airplane made out of that stuff?”)

The first modern flight data recorder, called "Mata-Hari", was created in 1942 by Finnish aviation engineer Veijo Hietala. This black high-tech mechanical box was able to record all required data during test flights of fighter aircraft that the Finnish Air Force repaired or built in its main aviation factory in Tampere, Finland.

Then, in 1953, while working at the Aeronautical Research Laboratories (ARL) of the Defence Science and Technology Organisation in Melbourne, Australian research scientist David Warren conceived a device that would record not only the instrument readings, but also the voices in the cockpit.

The disappearance of Malaysia Airlines Flight 370 demonstrated the limits of the contemporary flight recorder technology, namely how physical possession of the flight recorder device is necessary to help investigate the cause of an aircraft incident. Commentators have called for live streaming of data to the ground. To quote Stephen Trimble from his opinion piece in The Guardian on 10 March 2014:

“The ongoing mystery of Malaysia Airlines flight MH370 is the fault of a bizarre quirk in our networked society. Even cars have broadband connectivity now, but the modern jet airliner – perhaps our most technologically evolved mode of transport – still exists in the age of radio.”

Despite this technological shortcoming, black box recorders have played crucial roles in numerous aviation accident investigations, providing essential data that helped determine the causes of crashes, leading to changes in training, technology and regulations.
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References

https://en.wikipedia.org/wiki/Flight_recorder
https://www.nma.gov.au/defining-moments/resources/black-box-invented
https://www.theguardian.com/commentisfree/2014/mar/09/malaysia-airlines-flight-mh370-black-box

Images

1. "Black" box flight recorder
2. Black box elements
3. "Mata-Hari" flight data recorder. Photo credit: Marika Tamminen for Vapriikki Photo Archives
4. Dr David Warren with the first prototype flight data recorder. Credit: nma.gov.au
5. Malaysia Airlines Flight 370