According to the University of Minnesota, "Water soaked areas at the blossom end of the fruit usually appear when the fruits are one third to one half full size. This enlarges and darkens as the fruit matures. These large sunken lesions dry out flatten and become black and leathery. Typically the first fruit are most severely affected, and later developing fruit can be unaffected.
Causes and the role of calcium:
Blossom end rot is a "physiological disorder" induced by a localized calcium deficiency in the fruit. The incidence of the disorder is usually not due to a lack of calcium in the soil, but rather due to factors affecting the uptake and translocation of calcium. On a cellular level calcium is a critical component of cell walls (a structural nutrient). So when calcium is limiting cell walls cannot form properly and rapidly growing parts of the plant suffer breakdown. As a structural component calcium once incorporated into a cell wall is not mobile within the plant."