Cats can eat blackberries safely in small amounts. Blackberries are not toxic to cats and can provide some antioxidants, fiber, and vitamins, but they should only be given as an occasional treat and not as a substitute for meat. It is important to serve blackberries in small, bite-sized pieces to avoid choking, and to wash them thoroughly to remove any pesticides or chemicals. Cats may or may not like the taste, as they generally do not perceive sweetness. Overfeeding blackberries or any fruit can cause digestive upset due to cats' limited ability to process plant-based fiber and sugars. Moderation is key, and all treats, including fruits, should make up no more than 10% of a cat's daily caloric intake. It is always best to check with a veterinarian before adding new foods to a cat's diet.