Blossom End Rot is caused by a lack of Calcium at fruit set and that lack has multiple reasons. It might be that your soil lacks sufficient Magnesium and the plants cannot then utilize the Calcium they have so maybe, possibly the Espom Salts might help. Maybe the plants did not have enough available moisture at the time and could not move the Calcium up the stems. An excess of Magnesium can also cause the plants to not be able to use the available Calcium, so applying Epsom Salts might also be bad for your plants.
The best way to try to ensure your tomatoes do not get Blossom End Rot is to be sure the soil nutrients are in balance and that soil will be evenly moist but well drained.
Perhaps this article be Charlie Nardozzi might be of some help.

Here is a link that might be useful: When good tomatoes go bad