Kombu or konbu is dried kelp. This sea vegetable is available in the form of hard, flat, black sheets that have a fine white powder on the surface. Kombu is used to flavor dashi, a basic soup stock, and sushi rice. Wipe the surface of the sheets with a damp cloth before using to remove the powder: do not wash the kombu as you will diminish its flavor. Avoid the kombu that is wrinkled and thin.