I thought that this article on WebMD was interesting.
In the study, presented at Experimental Biology 2009, researchers fed rats bred to become obese either a high-fat or low-fat diet enriched with whole blueberry powder or carbohydrates as 2% of their total diet.
After 90 days, the rats fed blueberries had less abdominal fat, lower cholesterol, and improved glucose control and insulin sensitivity. The latter two factors are markers of how well the body processes sugar for energy and are related to diabetes risk.
These health benefits of blueberries were evident in rats fed both high- and low-fat diets enriched with the blueberry powder. But the benefits were greatest among those who ate a low-fat diet.
Although more research is needed to confirm these results in humans, a related study presented at the same conference showed that men with risk factors for heart disease who drank wild blueberry juice for three weeks seemed to experience slight improvements in glucose and insulin control.