Answer: Based on your description, your tomatoes are suffering from blossom end rot or BER. This is caused by a lack of calcium but more commonly by uneven soil moisture levels. Tomatoes need soil that is evenly moist meaning damp like a wrung out sponge, it should not be sopping wet/saturated or dried out. Watering correctly will prevent BER. Using an organic mulch several inches thick will also help keep the soil more evenly moist. In the meantime, the affected tomatoes are still edible -- cut off the black portion and the rest is still edible.
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