Every year I try to grow tomatoes with little success. The undersides always turn flat and black.
Blossom end rot is a deficiency of calcium. The problem starts at the bottom of the tomato as a pale, brown spot that turns black and flattens the bottom of the fruit.
A condition that seems to contribute to blossom end rot is irregular levels of moisture in the soil. Tomato plants take in nutrients, including calcium, through moisture. Inconsistent watering deprives them of these nutrients. Water regularly, every four to seven days is usually sufficient. During droughts or if you are gardening in containers you may need to water every day. To prevent leaf diseases, water low, keeping the foliage as dry as possible.
To keep the soil consistently moist, cover it with a two to three inch layer of mulch. Keep the mulch away from the stem of the plant. You'll find this also helps keep weeds down.