Blossom-end rot is caused by insufficient calcium in the tissue of the tomato. Calcium is taken up into the plant through the roots, however, it settles in one part of the plant. This means that the rot can occur even when there is an ample supply of calcium in the soil, stems or leaves. Actively growing parts of the plant such as developing tomatoes must have a continuous supply of calcium to prevent these spots from developing.

The conditions that cause blossom-end rot are closely linked to inconsistent soil moisture throughout the growing season. Since calcium is only moved into the plant with an ample moisture supply, when drought occurs the fruit continues to develop but will be affected by a calcium deficiency.

Rapid early growth of the plants can cause the rot because the calcium is needed by the tomatoes when they are actively growing and the plants may not be able to take up sufficient calcium quickly enough through the roots.

Mulching tomatoes helps to conserve moisture in the soil. Use organic mulches such as straw free of weed seed, grass clippings, peat moss or wood chips.

Excerpted from, and for more information:

https://www.canr.msu.edu/resources/blossom_end_rot_tip_sheet