What to do with black radishes?

I'd suggest two ways, but taste a sliver to decide. If they are not too stout- I like them shredded and in a grated salad along with carrot, green onion, and an "Asian" inspired sesame oil/ginger vinaigrette. They are also pretty good simply roasted, coated in olive oil, salt and pepper. If the skins are really thick and tough, they will slip off after roasting- and you can mash the white insides up with butter. In fact, you can mash them along with roasted potatos, and that's pretty yummy too - but without the skins, since that makes for a real unappetizing color scheme in the final mashup.