Black garlic is a well known Korean ingredient that has been used for over 4000 years, it's originality is surrounded by some controversy over who first started using it as the Japanese and the Chinese both claim it as their invention but with their countries being close together and combined with a history in invasions, it's likely we will never know but Korea seems to have taken 1st place in the eyes of the internet.
Black garlic is a product of time, it is not quick to make and if you try to speed up product then you forgo the desired texture and the unami flavour bomb that comes from the maillard reaction being given the time it needs. This leads onto my next point, the common response to people asking how it's made many people say it has been fermented which isn't the case, the colour and the flavour are made from a process known as the maillard reaction, this is a chemical reaction between amino acids and the sugars within the cloves with the addition of a steady constant heat over a particular length of time but not hot enough to burn, the common name for this process is caramelisation.
It is only recently that black garlic has made it's escape from Asian cuisine and has been experienced by the western world, where experimentation has lead to a miriad of uses for it. Because of how the flavour is so significantly different from White garlic black garlic can't be used as a replacement as you would need to use a significant amount of black garlic to reach the same intensity a smaller amount of white garlic would provide and instead black garlic is used to add additional subtle flavour notes such as umami. The Subtle notes that black garlic provides can bring an extra level of flavour to many dishes you may not think of such as mayonnaise or soup but if you want it to the prominent flavour then something like mashed potato would very much highlight the savoury but also acidic flavour.
As it's use is only increasing as is different ways it is presented, ranging from in ketchups, vinegars and as a salt.