I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
The problem is most likely blossom end rot (BER). It is caused by a lack of calcium to the tip of the fruit. When the cells in tip of the tomato lack calcium during growth, they die and the black decay you see follows. You may have adequate calcium in the soil but still get BER due to soil moisture fluctuations (from wet to dry). I have noticed that BER is worse on the first tomatoes of the season and tends to not affect later fruit as much.
If you haven't had a soil test in the past few years, it would be a good idea to have one done to make sure your calcium levels are adequate. Your County Extension Office can assist you in having your soil tested.
BER is best prevented by keeping soil moisture evenly moist. If your soil is sandy, and prone to moisture fluctuations, adding compost prior to planting may help. Garden centers sell a BER spray (contains calcium). This must be applied starting when tomatoes are about marble sized in order to prevent BER on the fruit.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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