Pomegranite seeds are covered with the juicy edible portion that is characteristically rather sour. The hard seed portion will be bitter, but most people spit out that part when eating the juicy fruit portion.
Sweetness of the fruit may be related to the genetics of the variety ahd/or to the soil fertility and water management . You can't change the genetics, but you can improve soil fertility which provides nutrients for production of sugars. Use a complete organic fertilizer like this, spring and fall:
Adequate watering is essential to support the metabolic processes that create carbohydrates in the plant tissues and fruit. Deep water pomegranite at least twice a month during your Concord hot dry season, May-June through October. Mulch the soil surface to help retain soil moisture. Drip irrigation or light sprinkling can keep trees alive but is not enough for optimum health.