Mystery kitchen reaction

So, I went back to my parents’ place for the past couple of days. Before I left, I had cleaned a wooden cutting board (the wood is walnut) and a Chinese-style stainless steel cleaver and put the cleaver on the cutting board. The cleaver had been dried a bit, but likely had some moisture remaining.

When I got back today, the portion of the cleaver touching the board had somehow reacted with it. The area that it was touching the wood has little specks of rust on it, and the wood itself has turned a pitch black. The rest of the cleaver is otherwise unblemished, as is the rest of the cutting board.

Anyone have any clue as to what might’ve happened here?

Are you sure it isn’t carbon steel? Chinese cleavers often are.

It could well be. If it is, it’s a relatively resistant type, though, since I know I’ve used it and abused it with no serious staining problems before.