Click below to listen to my Garden Bite radio show: Blossom end rot
A calcium deficiency will bring about blossom end rot. Uneven watering is one of the biggest culprits. There are foliar sprays but they only help BEFORE the tomato has blossom end rot, and, as you’ll see from the University of Minnesota, there’s disagreement on how well they work.
If you start to see this problem, remove the affected fruit, spray the rest of the plant.
Give your plants plenty of room. Best Practice is to give a good 4 feet for each plant. Keep as evenly moist as you can. The hardest thing on a tomato plant is to let them dry out like the Sahara and then douse them with water like a Fargo flood.
Here’s a bit more information from the University of Minnesota Extension on Tomato Blossom End Rot