Black Garlic isn't really fermented garlic. The process involves a slow sugar break down over time. Black garlic is heavily caramelized garlic.
I wash and sterilize the container of my rice cooker, then put heads of garlic upright in the bottom, spray the garlic heads lightly with water, close the lid and turn the cooker to the “warm” setting and let the garlic process for at least 14 days. After 7 days, spray lightly again with water, close the lid and keep the rice cooker running at “warm”. The generally accepted processing time is 40 days, but I usually pull my black garlic after 14+ days, but not 40 days. I also leave the rice cooker in the garage, as the smell can be pretty strong.