Blossom End Rot (BER) is a common problem with plants in the nightshade family, which happen to be some of the most popular garden plants. These include tomatoes, peppers, and eggplant. BER is not a fungal or bacterial disease, but rather a calcium imbalance.
We just lost two poblano peppers due to BER. It’s kind of funny, because none of our other plants have had the problem and we used the same potting soil on all of them. That being said, I’m going to address the problem using pelletized calcium. To prevent BSE, being consistent with your watering is very important. This affects the way the plant is able to uptake calcium in the soil. Also if you are fertilizing, not going with a balanced option but rather something that is lower in nitrogen and higher in phosphorus is helpful, as nitrogen may get in the way of the calcium uptake process. We did that this year with an organic fertilizer formulated specifically for nightshades, but for whatever reason the fruit on this one plant are affected. When you see an affected fruit, remove it immediately. There is no saving a fruit once it begins to rot.
As I mentioned, I will use pelletized calcium on the affected plant because I happen to have some on hand. There are liquid calcium solutions, and even formulas specifically for addressing blossom end rot. You could even use water that you boiled eggs in to water the plant, after it cooled down of course. Anything you can do to give the plant a boost of calcium can be beneficial in helping to prevent BER in other fruits.
I certainly hope you don’t need this info, but if you do, there it is!