Blossom End Rot
Leathery dark-coloured patches appear at the base of the fruit. This can be more of a problem when growing tomatoes in pots or in grow bags.
Ensure that tomato plants are kept reguarly fed and watered. Do not allow the compost to dry out. Blossom end rot is symptomatic of dry conditions and the plants not being able to move nutrients up and into the plants system.
There is no product available to stop blossom end rot.