Blossom end rot is a common nutritional disorder in tomatoes, peppers, eggplants, and squash that is caused by a shortage of calcium in the growing fruit. This nutritional disorder typically occurs when plants are growing rapidly and often affects the first developing fruits. Dark blemishes appear on the blossom-end of affected fruits as cells break down. They may enlarge until the entire bottom of the fruit becomes dark, shrunken, and leathery. Factors that encourage blossom-end rot include low soil pH and low levels of calcium, inconsistent watering, shallow watering or droughty conditions, and excessive use of nitrogen fertilizers.

To help prevent blossom end rot maintain the soil pH in the 6.3 to 6.8 range. A soil test kit from your local extension office will test for pH and other nutrients, Mix in a small handful of ground limestone into the soil around each tomato plant. Keep plants well mulched and watered through the growing season. Water deep at least once per week if rainfall is lacking. A mature tomato plant may require 2-3 gallons of water per week. Do not use high nitrogen fertilizers on your tomatoes.

The following article has more tips to prevent blossom end rot on tomatoes. The article can be found here: Blossom-end rot of tomato tip sheet - MSU Extension

Thank you for your question and have a great gardening summer.