Blossom end rot seems to be a big problem this season. A sunken black or brown decay will form where the blossom was on the bottom of the fruit. This is caused when the plant can't get the calcium it needs. The nutrient is probably in the soil, but unavailable when the soil dries out. The way to beat blossom end rot is to keep the soil evenly moist. Don't let the plant go through periods of drought then to be deluged in a summer thunderstorm. A good thick layer of mulch helps.
Some varieties of tomato are more susceptible than others, often sauce or paste tomatoes are hit hard. Tomatoes grown in containers are also prone to the problem. They need constant watering or you might consider a self watering container. This video explains how to build one.
But it's not just tomatoes that gardeners need to be worried about, eggplant, peppers and zucchini all can fall victim to the affliction.
I've had countless gardeners calling me, worried that late blight has hit their tomatoes, when in actuality it's only blossom end rot. As always, identify the problem, and then take action.