Blossom end rot is not caused by pests,over watering or any thing you have done. It is more what you didn’t do. You can tell you have blossom end rot on any fruiting plant if the fruit has a soggy looking spot on the end of the fruit where the blossom was. The soggy spot will turn black and look rotten. Blossom end rot happens when there is not enough calcium in the soil. For tomatoes, what I do to prevent it is when I am putting the tomato seedlings into the garden I add a couple tablespoons of previously washed and crumbled egg shells and I give each newly planted tomato seedling a cup of milk that I make with powdered milk mixed with water according to the directions on the package. The milk gives them calcium immediately and the egg shell will break down through the summer to supply calcium all through the growing season. Since I have been planting my tomatoes this way I have never gotten blossom end rot. It works.
You can see the blossom end rot on the pepper to the left.