hello sounds like blossom end rot caused by a lack of available calcium in the soil http://vegetablemdonline.ppath.cornell.edu/factsheets/Tomato_BlossRt.htm
I agree with Pierrot, definitely sounds like blossom end rot. Although this is caused by lack of calcium, the most common direct cause is insufficient water. There is probably plenty of calcium in the soil, but it will not get into the plant if there is insufficient water. I would gradually increase the amount of water your plants get - this will usually solve the problem. If you rapidly increase the water supply you will get a new problem - split fruits! Good Luck. BrianO
