Poke berries can cause vomiting and diarrhea particularly when eaten raw. The entire plant can be toxic. Typically the greens and roots are more toxic than the berries.
This plant is listed as a toxic plant as it induces vomiting and gatric upset.
The purple by product of the dog being ill can stain things terribly and the berries of this plant are used as natural dye sources.
If your dog seems really ill he or she may need vet supportive care for toxin ingestion or vomiting and diarrhea and if the dog ate a lot of it I'd see your vet for help as an overdose might be deadly.
Hope this helps you!