Experts and BHG readers answer.
Black spots on the bottom of tomatoes.
This is a condition called Blossom End Rot and it's caused by inconsistent watering. If it rains and then dries a lot, or if the plants are watered frequently, but shallowly, this will cause the condition. The best way to prevent this from happening is to keep your tomatoes well mulched. This will maintain a more consistent soil moisture and will lessen the problem with Blossom End Rot."