A squirrel’s diet consists primarily of nuts and berries as well as miscellaneous other foods it finds, so it is okay to feed them blueberries as long as you do so in moderation. You will definitely want to keep the number of blueberries you feed squirrels to a minimum, as too many is not good for them.
If you feel the need to feed your squirrel blueberries, you should not give them more than three or four at a time. It is important for these animals to get a variety of foods in their diet so they can get everything they need to stay healthy.