Blueberries have a natural defense system on their skin to help make them resistant to mold. The skin has a high wax content.
In this picture, notice the little white rings? It looks like white bloom, similar to what happens to chocolate when it is temperature abused. Those white spots are melted wax from the blueberries that melts at a temperature of at least 140°F. When canning fresh blueberries, the wax melted, then re-solidified during cooling leaving the little white rings. Reheating should melt it again.
This is not harmful and is a natural component of the fruit.
Source: Oregon State University Master Food Preservers
