Black plums are a round fruit with a thin, smooth blue-black skin. The interior flesh is a dark red, purple or yellow with a sweet/tart flavor.Plums and prunes are rich in antioxidants, which are helpful for reducing inflammation and protecting your cells from damage by free radicals. They are particularly high in polyphenol antioxidants, which have positive effects on bone health and may help reduce the risk of heart disease and diabetes