Symptom : it's a common problem with homegrown tomatoes. It initially appears as a depressed, leathery, brownish area the size of a dime to a half dollar on the blossom end of the fruit.
Cause : It is caused by a calcium deficiency coupled with wide fluctuations in available moisture. Training and pruning the tomato vines may also increase blossom end rot.
Treatment : Remove the affected fruit so other fruits on the plant will develop normally. Keep the plants well watered. Mulch to maintain a more uniform moisture supply. Avoid cultivating, or hoeing, near the roots of tomato plants. Some gardeners like to add crushed eggshells to the transplant hole to fortify calcium intake.
You can also use a Blossom-End Rot Spray to help correct calcium deficiency in tomatoes.