Blueberries are an excellent source of fiber, vitamin C, and antioxidants. They are generally a safe treat for cats.
This being said, you should not give your cat blueberries too often, or give too many blueberries to your cat at a time.
Having too many blueberries can lead to your cat having an upset stomach. Do not give your cat more than one or two blueberries every couple of days. If your cat eats too many, it will experience loose stool.
It is actually becoming a trend to use extract from blueberries on cat food. This is because blueberries have more antioxidants then most other fruits, and these help cat food stay fresh longer.
If your cat food has blueberries in it, it will be much healthier than others. This is because the cat food will use the antioxidants form blueberries to preserve the food instead of salts, or un-natural chemicals.
Blueberries are also very high in Vitamin C. Although cats do not need a lot of vitamin C to survive, it helps to reduce cancer rates. (Cats that have had urinary calcium oxalate stones should not have blueberries, as the calcium can increase the chance of having one)
There is a slight possibility that your cat may be allergic to blueberries. Although it is very rare, if you have never given your cat a blueberry before, only give one, and watch your cat for strange behaviour. If you cat displays symptoms such as sneezing, coughing, or acting strange, do not give it any more blueberry.
Cats usually eat blueberries like this: