Blueberries are not in fact bad for cats and you will find that many of the higher quality cat foods will include them for flavor.
There are certain foods that you should avoid feeding your cats though, such as grapes and raisins. The dried out nature of raisins makes it difficult for cats to eat and digest them, as with grapes because of their skin.
You can feel free to feed your cats blueberries; it might be a good idea to put them in with their regular cat food so they will be able to enjoy them even more. Blueberries definitely make for a great way to change up the normal flavor of any cat food.