Elderberry Umeboshi? Not really

In season 22 of Top Chef ™, Massimo mentions umeboshi made from elderberry, but strictly that's not a thing.

Traditional umeboshi are Japanese pickled plums (ume fruit), not elderberries. Ume are a specific type of Japanese apricot-plum, and authentic umeboshi are made exclusively from these fruits, salt, and often red shiso leaves.

Elderberries, while used in various culinary applications, are not typically used to make umeboshi-style pickles. Elderberries are more commonly used for jams, syrups, wines, and baked goods in European and American cooking traditions.