A dog vomiting blood can be caused by a variety of reasons and is a worrying symptom. Some possible causes include:

Internal trauma, such as an accidental injury or swallowing a sharp object, may cause damage to the stomach or intestines and vomit blood.

Digestive system diseases, such as gastroenteritis, gastric ulcer, etc., may cause gastrointestinal bleeding.

Poisoning, certain toxic substances may cause damage to the dog’s digestive system.

Liver or kidney disease, which affects blood clotting and digestive function.

Blood disorders, such as thrombocytopenia, may cause clotting problems.

Infections, such as bacterial or viral infections, may also cause gastrointestinal problems.

Tumors, tumors in the digestive tract may cause bleeding.