If you’ve got tomatoes that look like the above pictures, you’ve got Blossom End Rot.
Now, this isn’t a disease or insect damage or anything terrible like that but its to do with the way you’ve been treating your plants and is recoverable so don’t despair.
The actual cause is lack of calcium but what you’ve probably done wrong is uneven watering. I don’t know why but tomatoes like to be watered evenly throughout the season, keeping them damp down to a depth of about six inches is what you’re trying to achieve. In fact you’re best to water them from below, encouraging roots to grow as deep as possible (but of course you can’t do that if you’re growing in the greenhouse border or growbags which is why I tend to grow in pots filled with growbag compost).
Different varieties are more susceptible and we’ve found that long, paste, Italian varieties are more likely to succumb than round and beefsteak varieties (however, as you can see from the pictures above, we had problems with White Zebra in 2018).
As well as uneven watering, there is a suggestion that putting the plants out in cold soil could contribute to the problem so it will probably go away later on in the season.
There isn’t a lot you can do about it, try to make sure that you are watering evenly, throw the affected fruit away and hope for the best.