The sign said the garlic was sweeter than white garlic and had a taste similar to chocolate. It boasted: No garlic breath! I asked the man ringing me up if he knew anything more, and he said they're soft and slightly sweet and you could can actually eat them as is. He also said they'd been selling like crazy.
Curious, I returned home do some more research and learned that black garlic isn't a different variety of garlic; it's simply the result of fermenting it. Because garlic already contains sugars and amino acids, those are amped up in the fermentation process, which produces a new black garlic with molasses undertones and a rich savory/sweet finish.
Related: Quick Tip: Freeze-Roasted Garlic