Blossom end rot is caused from a calcium deficiency due to irregular watering. When planting your tomato plants use a good compost additive and a couple tbl spoons of epsum salts (magnisum sulfate) in your soil mixture. after planting mulch with hardwood mulch 2" thick 20" dia. This will help keep the soil moist around the plant. Epsum salts help the plant absorb calcium. Keep your plant watered thru out the season. Not wet but never dry. Be consistant. Top dress epsum salts once every 3 weeks (a coulple tbs spoons).