Blossom end rot is a name given for a disfiguration in the fruits of several plants and is a common physiological disorder. It starts as a small, wet looking spot at the blossom end of the fruit. Over time, it increases the surface area it covers, eventually leading the part of the fruit affected to darken with brown or black color and become stiff. It is not a disease nor is it a result of insect activity.
Blossom end rot happens when something goes wrong in the growth process. Part of the cells within the fruit stop functioning, turn black, harden, and eventually die. The occurrence is usually blamed on a shortage of calcium, but things are a bit more complex. While it is true that fruits that exhibit blossom end rot symptoms are found to be low on calcium, soil calcium shortage is not always the cause for this disorder. The problem is more often related to the mobility and availability of calcium inside the plant.
Blossom end rot can occur at any time, but tends to be more common at the beginning of the growth stage, appearing on the first fruits of the season.