I have tasted this garlic for the first time a few years ago. For many it is new and the black color comes from the fermenting process that has been around for many thousands of years with its origins from Korea.
Garlic contains sugars and amino acids. When garlic undergoes fermentation, these elements produce melanoidin, a dark-colored substance that is responsible for the color of black garlic. I measure the color on the lovibond @ 50+ on the scale: http://en.wikipedia.org/wiki/Beer_measurement. These Melanoidins are desirable flavor compounds that are especially present in German malts like Munich and Vienna. It is almost sweet yet has a punch of garlic. It can be consumed as it is and never look back.