Hold the whole, unpeeled garlic heads at a constant temperature of 130 degrees F for 2 to 3 weeks until the garlic is soft and black as tar.
Store the blackened garlic in an airtight container at room temperature for a month, or in the refrigerator for 3-6 months.
Optionally, dry the black garlic by peeling the cloves, then slicing thinly. Dehydrate at 125 deg F until brittle, about 12 hours. Grind the dried slices into coarse flakes, then return to the dehydrator for another 8-12 hours - you want the flakes to powder without clumping when ground.
Store black garlic powder in an airtight container in a cool, dark spot.