This is Blossom End Rot. Regular watering will take care of this, and no matter what anyone tells you, amending with calcium won't solve the problem (Unless your soil is ABSOLUTELY piss poor sand, in which case it might be bereft of calcium).
The only remedy to BER is regular watering, which will promote regular uptake of calcium.
The only remedy to BER is regular watering, which will promote regular uptake of calcium.