What causes blossom end rot on tomatoes.
Blossom End Rot on Tomatoes
Two things - in theory!
1. Uneven watering, allowing plants to dry out and then trying to recompense.
2. Watering from the top and wetting the fruits that have already formed. Try to water at the base, and to help polination spray the open flowers with a fine mist.
If your blossom end rot is not too bad, you can always try slicing the tomato horizontally and therefore cutting off the rot.