This article courtesy of Ohio State University Extension Services.
More information at www.bygl.osu.edu.

Blossom end rot is a common problem on tomatoes, especially early ripening ones. This problem also affects plants in the squash family, particularly summer squash, as well as eggplant, peppers, and even some melons. Just near the time fruit ripens, a dry sunken decay develops on the blossom end (terminal tip) of the fruit. This is not caused by an organism but rather a physiological disorder associated with a low concentration of calcium in the fruits.

Hold on however, before applying calcium to the soil. Unless a soil test has been done and low calcium levels are reported, don't apply calcium to the soil. In many cases, there might be plenty of calcium in the soil but it may be bound up in the soil, or is unavailable to the plant due to drought stress or excessive soil moisture fluctuations. This reduces the uptake and movement of calcium into the plant.

Foliar applications of calcium are often recommended. While one might think this works, they are of little value because of poor absorption and movement into the fruit where it is needed. The application of Epsom salts is also recommended but this does not remedy the situation either. Avoid drought stress and wide fluctuations in soil moisture by watering the plants in the morning, two to three times a week. This practice may lessen the problem, but will not totally solve the problem.