They have blossom end rot. This is caused by inconsistent watering/ moisture levels in the soil.... you need to water them more frequently. If you have 2 plants in one of those containers you have to water at least once a day if not more when it is really hot.... or as the plants get bigger and require more water.

Sounds like blossom end rot - BER for short. It is caused by a calcium deficiency, although some gardeners say it can be caused by too much watering. Do a quick search and you will find plenty of posts and FAQs.

Earlier in the season when it was very rainy and wet, I was getting a lot of BER mostly on romas. But it is not happening anymore, because its dry and hot and always water my plants from base.

With upside downs, the water will run over the flowers and fruits. And if it does not dry up quickly, may cause BET.
So, it would be better to water it in the morning. If watered at night, it will not dry all night and the bacteria (or watever is rsponsible) can thrive and work and establish itself.

spooky! was going to ask the same question about my plum toms! not in upside down planter but for what every one is discribing, sound like a watering problem and deficiency with mine due to there depth and position, thanks

Here you go.

Here is a link that might be useful: BER FAQs

Blossom end rot does usually come right early in the season or then late summer

Here is a link that might be useful: Blossom-End-Rot-of-Tomato