Unfortunately, it sounds as though you have Blossom End Rot, caused by lack of calcium, irregular watering/allowing to dry out. Remove any fruit which have this, be regular with feed and watering. They should recover, it is not a disease.
you have to see where is the black bloth appear. At the blossom end of the fruit?, applying lime or calcium before planting. using excess amount of nitrogen also happen this. Root injury also happen this. Are there any character, need to analyse more.