Answer: Your tomatoes have "blossom end rot" (BER), a physiological condition caused by a lack of calcium at the growing tip of the fruit. It is also a common problem on peppers. While your soil may have adequate calcium, fluctuations in soil moisture content from dry to wet really increase the incidence of BER. It is especially bad on the early fruit each summer and in sandy soils.
The damage occurs as cells die at the tip of the fruit. In time (and as the fruit grows) the spots enlarge and turn black. So, by the time you see it, the damage actually has already occurred some time back.
Remedies for BER include having a soil test to make sure calcium levels are adequate, adding organic matter to a sandy soil to increase its moisture holding capacity, and keeping plants evenly moist, especially during the development of the first fruits. A thick mulch will help maintain soil moisture.
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