You can absolutely make your own black garlic. All that is required is to have the garlic in a vaguely air-tight container (preferably individual wrapped or contained) for 30 days at 140°-155°F. My method, covered at my blog, is to put the garlic in mason jars in my light bulb heated black garlic oven, which can be made for about $30 and can ferment 12 bulbs easily for about $4 of electricity over 40 days. With the cost of garlic, glue, aluminum foil and the mason jars, the oven with one recipe is less expensive than an equivalent order would be online.
The first 30 days, the jars are kept sealed in the mason jars, to allow the garlic to ferment properly. After 30 days, the lids are removed and the heat in the box dehydrates the garlic, leaving a dried out head with shriveled black cloves inside of it. The flavor is reminiscent of balsamic vinegar and soy sauce, but without the sourness of vinegar or the saltiness of soy sauce. The oven design is easily doable for anyone of any DIY skill level with access to a hardware store and an Academy (where they sell the Huskee cooler I use for $25).
YES. Yes, you can. I just did.
My first attempt in my rice cooker had uncovered garlic heads in it for 10 continuous days-- the result was black but dried hard as rocks (and the process stank up the entire house the whole time).
My second attempt involved wrapping each head in two layers of foil, then putting the garlic in a much lower temperature warming oven (no thermostat at all, just a light bulb) for 10 days, which resulted in beige garlic. I then switched it over to the rice cooker for 2 days, turned it off, checked them, then another 2 days, turned it off, forgot about it for a day, checked them... another 2 days, turned it off, checked them... and found DELICIOUSNESS.
It needs more time, I think, it's not as sweet as I'd like yet and could be a little darker. I've seen some Koreans mention finishing the batch by just hanging the garlic in a bag for a week, so that's what I'm going to do.
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EDIT: Black garlic starts out as regular garlic and is put through a fermentation process that besides changing the color of the cloves to black, also mellows the flavor and reduces the sharpness associated with raw garlic. The texture of the cloves become very soft, also. While I have never used nor eaten black garlic, a website, linked below, is dedicated to everything related to black garlic, including how to use it in recipes.
Blackgarlic.com has a wealth of information on how this fermented version of garlic is made, its unique qualities including taste, texture and chemical properties, where to get it and...recipes!
http://blackgarlic.com/
NEW EDIT: And to balance out the linked info. so there's a non-commercial reference, I found this very in-depth article on black garlic's attributes that includes a reference to a scientific study: http://www.wellbeing.com.au/newsdetail/Black-Magic-Garlic_000553