Just as you’re going out to pick that tomato you’ve had your eye on for few days just waiting for it to get perfectly ripe,you’ve go it in your hand and turn it over to find-YIKES!!! The bottom is all black and ugly, what the heck? This is the time of year we begin seeing blossom end rot on : tomatoes, peppers, and even squash at times.” The disease is especially prevalent when rapidly growing, succulent plants are exposed suddenly to a period of drought. When the roots fail to obtain sufficient water and calcium to be transported up to the rapidly developing fruits, the latter become rotted on their basal ends”.Cornell University
So consistent watering and fertilizing is key and you can use a calcium spray made by fertilome as shown in the picture. A common mistake made by gardeners is to throw away the entire fruit. Simply cut off the affected portion and the fruit is fine to eat. This condition is also not spread from plant to plant or fruit to fruit so don’t worry about handling the fruit along with other non affected plants or vegetables!