Blossom End Rot (BER) is a physiological disorder of tomatoes, peppers, and cucurbits caused by a calcium imbalance within the plant. Fluctuations in soil moisture, excessively wet or dry soil, excessive nitrogen fertilizer, roots damaged by cultivation, very high or low pH, or soils high in salts prevent all can the roots from taking up enough calcium to satisfy the plant's rapid cell development. The result is a water-soaked spot at the blossom end of the plant that enlarges, turning dark brown and leathery. Rot may set in at the spot. BER is common when plants grow rapidly in the beginning of the season, then set fruit during dry weather. As little as 30 minutes of water deficiency at any time can cause BER.
Keep plants uniformly watered throughout the season. Water deeply; wet the soil at least 6 inches down. Apply mulch to maintain soil moisture. Keep soil pH around 6.5. Some older varieties of indeterminates (vining tomatoes) and plum tomatoes are more susceptible to BER -- make very sure they have adequate soil moisture.
Photo courtesy of David Liebman