It's blossom end rot. I happens due to poor flow of calcium around the plant. The reason that happens is due to inconsistent watering. If you can correct the watering, the problem will stop and when new fruits form they will not be affected. Are your plants able to drain properly?
Your tomatoes have bottom end rot It is usually down to irregular watering this does not mean your tomatoes are wasted .just slice off the black and use as normal.
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I happens due to poor flow of calcium around the plant.
The reason that happens is due to inconsistent watering.
If you can correct the watering, the problem will stop and when new fruits form they will not be affected.
Are your plants able to drain properly?
Some RHS info here
https://www.rhs.org.uk/advice/profile?pid=395
Wisdom is not putting it in a fruit salad.
It is usually down to irregular watering this does not mean your tomatoes are wasted .just slice off the black and use as normal.
Wisdom is not putting it in a fruit salad.