As per an earlier post;
Blossom end rot is caused by a calcium deficiency. This leaves soft tissue that often is infected by fungal spores, often resulting of total loss of that particular fruit. End rot is a totally physiological condition occuring most often in the early fruit. Mostly due to calcium deficiency but can also occur because plant has been hardened to fast, weather conditions are cooler during early growth (sometimes in late cooler summers).
"If your calcium is off add limestone (for acid soils with a pH below 6), or gypsum when the soil pH is in the 6 to 7 range. If calcium levels are okay, the next most important control is to maintain optimum soil moisture. When tomatoes experience the slightest bit of drought, BER may result. Using mulches will usually significantly decrease BER as excessive evaporation from soil is reduced. If growing on bare ground, avoid cultivating too close to plants to prevent root damage and the need to maintain deep root development. Varieties will vary in their susceptibility so if you have a problem with a particular variety, choose a new one next year. When side-dressing plants, using a nitrate type fertilizer like calcium nitrate is preferable to ammonium based ones like urea. Finally, don't bother to use calcium sprays. They are worthless in combating the problem. The same problem can occur on pepper and eggplant." As per Thomas A. Zitter, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY, and Steve Reiners, Associate Professor, Horticultural Sciences, NYSAES, Geneva, NY