According to About.com, blueberries are an acceptable food for guinea pigs. This fruit contains vitamin C, and it is essential that guinea pigs get an adequate amount of the nutrient. Otherwise, they develop scurvy, a serious disease.
The Humane Society stresses that, since fruit contains large amounts of sugar, it should be offered to guinea pigs in moderation. Guinea pigs should eat no more than several blueberries once a day.
There are other, less sugary ways to be sure that guinea pigs consume enough vitamin C. About.com indicates that vitamin C tablets for guinea pigs are available. Also, some commercial pellets contain vitamin C, but they lose potency quickly. Therefore, pellets are best consumed before the "use by" date stamped on the package.