It's through these extension offices the Master Gardener program originates, 4-H and many farm programs. When I don't have personal knowledge of certain information, I often use these sources. Their information is researched, accurate and tested.
The following is a blend of my personal experience and the fact sheets from the extension offices.
Blossom End Rot:
Blossom End Rot is a common disease this time of the year. Blossom-end rot is not caused by a parasitic organism but is a physiologic disorder associated with a low concentration of calcium in the fruit.
Calcium is required in relatively large concentrations for normal cell growth. When a rapidly growing fruit is deprived of necessary calcium, the tissues break down, leaving the characteristic dry, sunken lesion at the blossom end. Blossom-end rot is induced when demand for calcium exceeds supply. This may result from low calcium levels or high amounts of competitive cations in the soil, drought stress, or excessive soil moisture fluctuations which reduce uptake and movement of calcium into the plant, or rapid, vegetative growth due to excessive nitrogen fertilization. The problem is especially bad in hot weather.
Blossom-end rot is a dry, leathery brown rot of the blossom end of the fruit that is common in some seasons on tomatoes. The rot area sometimes looks black and this is due to secondary molds that affix to the rot.
Soil applications of calcium seldom help, though foliar calcium sprays may minimize the occurrence of the problem. Make sure the formulation is designed for foliar application or severe damage could result.
Pruning causes stress to the plants that may increase the incidence of blossom-end rot. Some tomato varieties are much more susceptible to this condition than others.
Mulching and uniform watering help to prevent blossom-end rot. Once the blackened ends appear, affected fruits cannot be saved. They are best removed and destroyed so that healthy fruit setting later can develop more quickly.
Blossom End Rot also can be present on peppers and eggplant - if you find it on your tomatoes, check these two, also.
I had to laugh at a comment I read recently: "Even though tomatoes have many things that can go wrong with them and are susceptible to many diseases and pests, it still doesn't keep the neighbors from putting a bag full in your unlocked car." Yes, we keep growing tomatoes because they are a part of our very summer experience and we'll keep eating them happily.