Yes, even one or two black grapes can be harmful depending on the dogs size and sensitivity. Because individual reactions vary widely, its safest to prevent any grape ingestion to avoid potential poisoning.
Overview of Canine Grape and Raisin Toxicity. Ingestion of grapes or raisins can be toxic to dogs.These signs of grape and raisin toxicosis in dogs generally can start within a few hours and progress over 24 to 48 hours.

Central nervous system (CNS). Brain sensory information. Initiates responses, stores memories, generates thought and ideas. Spinal cord conducts signals to and from the brain, controls reflex activities. Peripheral nervous system (PNS).

As we can see from the illustration, Black Grapes And Canine Nervous System Health has many fascinating aspects to explore.
(Equine & Canine streams). Learn how canine nervous system gathers, integrates & communicates information to support life.