It is Blossom End Rot (BER). It is a condition caused by lack of calcium, but almost never is it a lack of calcium in the soil. It is caused by stress conditions that make it hard for the plant to take up the calcium that is there.
Here's a thread about it:
https://www.helpfulgardener.com/forum/vi ... 15&t=25876
If the plants are put in to cold soil early in the season, that can cause it (the problem starts as soon as the fruit is set). But most often it is related to fluctuations in water and too much wet-dry cycles. Try to maintain consistent soil moisture - mulching helps with that.