FWIW,
I found a couple pics of Hunter's black garlic as well as a pic of black ramps on his Instagram.
In the comments on the black ramps there's evidence that baking soda is involved.
Baked baking soda — sodium carbonate — apparently.
"I was at your talk at the MOFAD, I just tried following your method for black garlic on ramps this week myself, didn't turn out this nice. Should I have baked the baking soda longer for a darker color? And in was a bit damp but you wrapped yours in a damp towel in plastic wrap right?"