Garlic turned to black powder?
Your garlic was moldy within it's peel, then dried out, leaving you with a "garlic mummy". This happens occasionally, sometimes with a single clove, sometimes with the entire head. 

According to the OSU Plant Clinic, there are different types of mold commmon on garlic, but most seem to be visible from the outside. In your case, the infamous Aspergillus niger seems to be the likely culprit, because it can enter though small wounds or even the neck when the leaves dry out, leaving the outer layers of the skin intact. It produces a large(ish) amount of black spores that look like a powder.