Thanks to the NC Cooperative Extension for this information:
Blossom-end rot is a fruit disorder that causes the blossom end of a tomato fruit to rot. The rot is dark brown in color and has a tough, leathery feel. It is usually worse on the first fruit cluster but can be a problem throughout the season.
Blossom-end rot is caused by a lack of calcium in the fruit. If the soil pH is too low, calcium is not available to the plant. Dry weather contributes to the problem.
Tomatoes that are already affect will continue to be deformed. They are safe to eat, but it reduces the amount of tomatoes that can be used. If you have a lots of tomato plants, it might be best to pick off and destroy the fruits that are severely affect.
You can also reduce blossom-end rot by spraying with a calcium chloride solution. Brand names include: Tomato Saver and Blossom-end Rot Preventer. Follow the label direction. Ideally, you should start spraying when the first green tomatoes are about the size of a silver dollar. Spray once a week for three to four weeks.
There are several things you can do to help prevent the problem from occurring. Having your soil tested to determine the pH. Incorporate the amount of lime recommended into the soil before you plant. This will greatly reduce the chances that your tomatoes will develop blossom-end rot. Apply a 2- to 3-inch layer of mulch after the soil has warmed up. Use either pine straw, compost, shredded newspaper, or wheat straw. The mulch will also help reduce foliar leaf diseases and conserve soil moisture. Water as needed to maintain uniform soil moisture. Tomatoes need about 1 inch of water per week. During hot dry weather they may need more. Blossom-end rot tends to be worse on staked tomato plants and when high rates of nitrogen fertilizer have been applied.