Blossom end rot disorder in tomato has been found to be caused by the deficiency of

Blossom end rot disorder in tomato is caused by the deficiency of Ca.
Blossom end rot is caused by a localized calcium deficiency in the distal end of the fruit.
Calcium is a relatively immobile element; calcium uptake is therefore heavily influenced by anything that affects water uptake by a plant.
It is required in relatively large quantities by rapidly growing fruit, especially by those cells at the blossom end of the young fruit.