Blossom End Rot Prevention and Treatment
Blossom end rot is caused by either a calcium deficiency (either a lack of calcium in the soil or the plants inability to take it up because of pH) or it can be environmental due to watering. For additional information on preventing or controlling, see the link below from NC Extension.
This second link provides information specific to growing tomatoes, with additional information on blossom end rot. It will also provide information on fertilizer, but I would highly recommend a soil sample to determine accurate nutrient needs. You can pick up the sampling kits at our office (301 East Center Street, Lexington).