Blossom End Rot
There is no shortage of Old Wives Tales and myths around prevention of blossom end rot (BER), that disappointing disease that turns the bottom (blossom) end of the tomato fruit black and rotten.
The basic issue at the heart of BER is a lack of calcium getting to the tissue of the developing fruit.
Although BER is caused by a lack of calcium, it is a lack of water that is most often responsible for its development.
As with most tomato diseases, prevention is the key.
Water your plants consistently. BER tends to happen after drought (or, in my case, "went away for a hot long weekend and the tomatoes didn't get any water").
Calcium is pulled from the soil and distributed throughout the plant through water that travels through the plant. including to developing fruit tissue.
Most soils in the Prairies are not deficient in calcium so resist adding lime or bone meal to your soil before you have done a soil test. In addition, the water in Alberta comes from runoff from the eastern slopes of the limestone rich Rocky Mountains, which is laden with calcium.
This tells us that a shortage of calcium is almost never the issue in Alberta gardens. It's inconsistent watering that prevents the plant from accessing all that great calcium needed for tomato tissue development.
Regular watering and well drained soil
Water the soil (NOT THE PLANT). The great key to a healthy tomato crop is keeping the leaves dry.
Rich soil with slow release tomato fertilizer mixed in at time of planting (or regular application of balanced tomato fertilizer such as 20-20-20)
Mulch the soil, either organic mulch or plastic mulch to help retain moisture
Testing soil and supplementing with lime IF the soil is calcium deficient
WHAT DOESN'T WORK
Sprinkling crumbled egg shells around the plants - the calcium in the shell is not bio-available to the plant and won't provide any calcium for several years as the shells get pulverized and slowly decompose into the soil
Epsom Salts - this is just flat out BAD for your soil. Epsom salts are magnesium sulfate (a form of magnesium salt). Soil in the prairies is not deficient in magnesium. Use compost or fertilizer instead. USask has a nice article on this with more detail: Epsom Salts are not recommended for tomatoes.
Powdered or liquid milk - the calcium in milk is bound to the naturally occurring fats in milk which means it is not available for the plant to use.
RESOURCES & REFERENCES
Clemson University, College of Ag, Forestry and Life Science: Tomato Diseases and Disorders
Michigan State University - Blossom end rot of a tomato tip sheet
Gardening at USaskatchewan - Growing Tomatoes
Salisbury Greenhouse - What's wrong with my tomato
University of Missouri - Hot Weather Tomato Problems
Cornell University - Tomato Disease Identification Key