Blossom end rot in tomatoes looks like this...
It appears on the blossom end of the fruit as it develops, causing major blemish and rendering the crop next to useless for marketing. We have experienced the rot on younger garden beds before compost and organic matter move the soil to balance. The cause is primarily due to the lack of calcium in the soil or may be due to stress from drought or water logging.
A key for us is the addition of ground eggshells in the compost. We save all of our egg shells and dry them for crushing and processing in a mortar and pestle. This last job often falls to wwoofers and interns on rainy days when the activities in the garden are restricted.
Another method is to add dolomite or garden or agricultural lime to the soil at the time of bed preparation. Mostly good compost at this time should be enough but young gardens may need a boost.
We have found that the appearance of blossom end rot may be rectified by dusting the plants with dolomite, just as a foliar feed of calcium. Be sure to cast handfuls of the dust from up wind and ensure you do not breath it in. The drift of the application will cover the plant with an added benefit in deterring fungal problems by altering the pH of the leaf surface.
Affected fruits are fed to the chickens