Blossom end rot. Cause is a lack of calcium. Next year either add some lime to the growing medium or spray the fruit with soluble calcium once the fruit has set.

I think it can also be caused by erratic watering but as I don't grow them (Him Indoors does) I can't be certain.

Drought can also be a cause, impairing the plants' ability to push calcium into the fruit. In pots, watch your watering. In the ground, apply a water-conserving mulch.

Thank you!