"This condition is called blossom end rot. It is caused by a lack of calcium in the fruit itself. There may be plenty of calcium in the soil, but it's not getting into the fruit.
Sometimes the first fruits are affected but not later ones because cold, wet soils can compound the problem. As temperatures warm up, the problem resolves itself. It can also be caused by fluctuating moisture levels in the soil. Mulching the plants to maintain more uniform moisture can help. It also helps to use a fertilizer with a nitrate nitrogen source rather than an ammonium nitrogen source. If your soil pH is low, you could add lime as a source of calcium."