bleach. if you seal it with poly, then every time you cut, carve ext, you will be chopping up polyurethane in your food

you should be using mineral oil on BB stations anyhow...

Why not replace it. Unless it is the whole kitchen?

Bleach overnight and it’s gone.

I think you need to talk to someone who deals with butcherblock. I don't know much about mold but do you know if the mold is going to stay contained or is it going to keep spreading? If it is going to keep spreading and you seal it in, then what would happen?

Another option is that you cut out that section and replace it. But I guess that depends on how big of an area is it, how visible is it and how unattractive is it, and you weight that against the visibility of a cut-in and two new seams.

You can also try Tilex, I think it's just a slightly different kind of bleach?

Did you try bleach? I'd also try Nature's Miracle (or a paste made of out Biz, both are enzyme cleaners) before I gave up. They might damage the wood but giving up should be replacing it anyway, not sealing it in.

I tried sanding it, but after several passes, small holes started being revealed where parts of the block come together, and there is still black showing in that area. Neither tea tree oil nor vinegar could get the remaining black off. I'm thinking about just putting mineral oil or some polyurethane (or low-VOC type sealant) on the surface now, to just try to seal the mold in. Bad idea?