Blossom end rot is caused by a calcium deficiency in the tomato. However, it is not because there is not enough calcium in the soil, but because the tomato plant can't take it up. And the reason for that is usually cultural or environmental: too much water, inconsistent water, too much fertilizer, cultivating around the plant, tomato variety, and so on. Read here:
For what it's worth, blossom end rot tends to affect mostly the first tomatoes to ripen and not so much the later tomatoes.