The ‘Black bottom’ is a frequent disease in tomatoes.
It appares either in green and in red fruit.
Actually it isn’t caused by microorgaisms or pests. It is the result of an irregular watering, an incorrect supply of calcium on the soil, and too deep difference between night temperatures and day ones.
Often the black area is damaged by other diseases as Phytophthora infestansor bacteria causing rot.
In my photo a red tomato affects by black bottom and late blight. It’s evident the rot area.
The diseases are appare when night temperatures below under 10 C.
It’s only possible to prevent. Add calcium in soil during tomatoes growing, watering regularly with drop sistem and cover with TNT plants to protect them from low night temperatures. It’s possible, also, growing tomatoes in cold glass house so to conditioning all these factors.