No, blueberries are not toxic to cats.
They are safe for most cats when given in small amounts as an occasional treat. Veterinary sources confirm blueberries pose no known toxicological risks and are not listed among harmful fruits like grapes or onions.

Potential Risks

Overfeeding can lead to digestive issues such as diarrhea, vomiting, or stomach discomfort due to the fruit's sugar and fiber content. Cats with diabetes, obesity, or sensitive stomachs should avoid them, as the natural sugars may cause blood sugar spikes or weight gain. Whole berries might also pose a choking hazard for kittens or small cats.

Benefits and Feeding Tips

Blueberries offer antioxidants, vitamins like C and K, fiber, and minerals such as potassium, providing minor nutritional perks without being essential for obligate carnivores. Wash them thoroughly, serve plain and fresh in moderation (1-2 berries), and introduce gradually while monitoring for reactions. Avoid processed forms like muffins, which contain harmful additives.