The disease, Tomato blossom end rot first appears as small brown spot at the bottom of an immature fruit. A secondary infection then spreads deeper in to the fruit.
There is no cure for blossom end rot, so remove the infected fruit and destroy teh. If left on the plant they will weaken the plant and spread the disease. Best not put your infected fruin in your compost heap as the fungal spores can be spread.
Blossom end rot is caused mostly by irregular watering or fluctuations in moisture and or by too little calcium in the soil.
Best to water regularly (in the evening to reduce water loss)
Don’t use excessive nitrogen fertilization.