The primary factor behind Blossom End Rot vulnerability in vegetables is soil moisture level and its fluctuations. Accidental over-watering and under-watering can impact how easily roots can absorb calcium in the soil, so generally the solution to the condition is to monitor watering more carefully, rather than having to apply calcium. More information can be found on the linked page above.

Are your tomatoes growing in the ground or in containers? Container soil dries out faster than in-ground soil, and laboratory testing the soil for in-ground plantings can reveal if there are any key nutrient deficiencies or an acidity level (pH) outside of what the plants prefer. (Testing container potting mix for nutrients and pH is usually not necessary and is a different type of test than tests run on soil, which might cost a little bit more from a testing lab.) If you have been applying a complete fertilizer regularly, a nutrient deficiency is not likely.

Miri