We had too much rain this Summer right on time when Tomato fruits started to set and I found that too much rain can cause some problems too, i.e. Blossom End Rot. It is not infectious and caused by chemical imbalance in the soil. Basically with too much water an essential nutrient Calcium gets leached out and causes Calcium deficiency when fruits are set and start growing. Calcium is playing important role in tomato fruit formation. So be sure you add enough of that into the soil where you plant tomatoes. Better off to add Calcium into the soil before transplanting seedlings outside, but  also as an emergency measure to save your tomato harvest - as soon as you noticed blossom end rot - spread  a handful of calcium around each tomato plant and water thoroughly. Any form of calcium will help: agri-limestome, bone meal, crushed/powdered eggshells or just crushed calcium tablets if you have only a few plants. Water your plants regularly so that calcium gets a chance to be absorbed by the plant sooner rather than later. This will remedy the problem going forward, but unfortunately cannot revert damage to already affected fruits.

I also found that just adding complex fertilizer specifically formulated for tomatoes to the soil at the time of transplanting tomatoes into garden will warrant that tomatoes go this disease-free.