Cause:

Plants move nutrients like Calcium (Ca) through a process called transpiration (plant sweating). As water evaporates from the leaves through the pores (stoma), it drags more water up through the roots like drinking from a straw. Calcium dissolves in this root water and hitches a ride upward. Calcium is said to be "immobile" meaning it cannot move through the plant by itself. 

So when the soil dries up, Ca is stuck in the soil.  Then, when you do water, the Ca channel moves again, but most goes to the leaves since they are more efficient at uptake.  This causes a deficiency in the fast growing bottom of the fruit, causing cells to collapse and turn black

How to Fix It

Try to water regularly. The best fix is drip irrigation which will deliver a consistent stream of water to the soil.  And water DEEPLY.  Drench each plant until water pools on the soil and stays on the soil for a few seconds. 

Before adding Epsom salts, consider this:  In plant biology, magnesium, potassium, and calcium all share the exact same root absorption channels. If you add Epsom salts (magnesium sulfate) to your soil hoping to fix your tomatoes, you overload the root channels with magnesium. The plant fills up on magnesium, completely pushing calcium away and drastically worsening your blossom end rot. So the best course of action is to use a foliar calcium spray, directly on the leaves, early in the morning when the leaf pores (stoma) are open.

Another great option is to amend the soil with gypsum.  Work a tbsp of gypsum into the soil around the stem, being careful not to damage roots or get too close to the stem.