GotGarlic
Chef Extraordinaire
So you didn't clean off the black but just used the other side? It's probably mold. I'd get rid of it and clean your boards thoroughly before storing.I can't remember what it was from, but should be from food. I don't use this board on doing anything otherwise than preparing food
I remember that it wasn't that black, and then I use another side, after some months, it gets blacker and blacker
My main cutting board is wood, for the reasons I described earlier.