Energy-giving foods are foods which not only supply lots of nutrition such as green veggies and non-acidic fruits but, also do not actually Drain any energy (Acids and Nightshades DRAIN energy and Dairy and Glutens/Gliadins drain energy in many people.) Blueberries are nightshade.
Energy giving foods, such as Carbohydrates, are necessary to biological life. They are necessary due to the energy requirements of individual cells to function. Without chemical energy (often in the form of Carbohydrates) your cells would be unable to function and eventually die.