Black plums are fine for dogs in small quantities.
Though some people may think they say “no”, there is no scientific evidence to support the claim that black plums are bad for pets. A good guideline is to give four plum pieces per day or one whole plum at a time. None of these should be eaten by humans [to prevent cases of toxicity]. If your dog becomes lethargic, vomits, has diarrhea, or has a seizure after eating a plum it’s important to call the pet’s veterinarian so symptoms can be evaluated and appropriate treatment can be instituted.