Cats can have blackberries in small amounts as an occasional treat. Blackberries are not toxic to cats and can provide some antioxidants, fiber, and vitamins such as C and K. However, since cats are obligate carnivores, fruit is not a necessary part of their diet. It is important to only give fresh, plain blackberries without any added sugar or flavorings, and to serve them in small, bite-sized pieces to avoid choking hazards. Too much blackberry or other fruit can cause digestive upset such as vomiting or diarrhea due to high sugar and fiber content. Overall, blackberries can be safely offered to cats in moderation, about one to two small pieces no more than once or twice a week, while monitoring for any adverse reactions.