Just today they sent a reprint of the article from Americas Test Kitchen regarding garlic presses. In answer to your question if "what's that black stuff?' I submit the following, copied directly from ATK's text. Hope this helps!!
<<As a side note, we also noticed that on many of the garlic presses we use in the test kitchen, the nonstick coating had peeled off each one in the test kitchen, particularly around the hopper; a tiny amount of black liquid was sometimes extruded along with the garlic. After some digging, we discovered that when the nonstick coating peels off, copper and iron in the aluminum base metal react with the air and sulfur compounds in the garlic to create oxides and sulfides, which we sometimes see as a black substance on our extruded garlic. It’s similar to the discoloration from an old-fashioned carbon steel knife, and while it’s not toxic, it's not very appealing. We downgraded those models.>>
<<As a side note, we also noticed that on many of the garlic presses we use in the test kitchen, the nonstick coating had peeled off each one in the test kitchen, particularly around the hopper; a tiny amount of black liquid was sometimes extruded along with the garlic. After some digging, we discovered that when the nonstick coating peels off, copper and iron in the aluminum base metal react with the air and sulfur compounds in the garlic to create oxides and sulfides, which we sometimes see as a black substance on our extruded garlic. It’s similar to the discoloration from an old-fashioned carbon steel knife, and while it’s not toxic, it's not very appealing. We downgraded those models.>>
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