Tomato Dry Rot
Have you ever cut into a tomato and found a black blob in the middle of your tomato? - That is most likely "Tomato Dry Rot" (see picture below). Before cutting into a tomato look at the bottom and if you see an unusual dark mark (as seen in the picture above) then that tomato will most likely have "dry rot." If the dark mark is severe enough I will just throw (compost) that tomato away but if it is a small dark spot then I will just cut around it - the rest of the tomato tastes fine.
Tomato Dry Rot is caused mainly by an extended dry period sometimes followed by an unusual wet period and it has to do with calcium absorption. Also, if there has been some root disturbance then dry rot might occur. I usually find that the first few tomatoes of every season will have some dry rot but then tomato quality for the rest of the season is fine. So, in other words - If you pick a tomato with dry rot don't pull up the plant - the plant is fine, just give it a little time and you will be harvesting perfect tomatoes. (Note: - I'm sure there is an exception to my rule but I have never had a tomato plant produce only dry rot tomatoes).