A friend of mine generously gave me a package of black garlic from Trader Joes. The entire garlic bulb is slow-cooked (for at least a month) and produces cloves that are soft like roasted garlic. The color is a deep dark brown: the color you'd expect from the inside of a vanilla bean or very good dark chocolate.
I finally tried a bit of it yesterday, and the flavor has notes that struck me as similar to balsamic vinegar - particularly the thicker and not quite so acidic Aceto Balsasmico de Modena, not exactly the stuff that was ubiquitous in the '90s.
I ended up putting a clove in with spinach and a bunch of fresh garlic that I'd sliced and browned in olive oil. Rather tasty.