From MSU Extension:
Blossom end rot is a perennial problem for many fruit and vegetable growers. Vegetables especially susceptible to blossom end rot are tomatoes, Roma-types in particular, peppers, eggplant, watermelon and summer squash.
Very simply, blossom end rot is caused by a lack of adequate calcium (Ca) in the fruit with the Ca concentration being lower at the blossom end compared to the stem end. Blossom end rot is more prevalent on early fruit than it is on later fruit and varieties can vary in their susceptibility. Calcium is an important nutrient needed by plants for cell wall development and other processes. Calcium is non-mobile in the plant, meaning that once it is in place in the plant, it cannot be moved to another area within the plant. Therefore, plants need a continual Ca supply for good growth.
On tomatoes, blossom end rot is often in combination with a black, sooty appearing fungus that is a secondary problem. The solution to minimizing blossom end rot lies in understanding the reasons plants might have difficulty in maintaining adequate Ca.
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