They have blossom end rot caused by a calcium deficiency with inconsistent watering. You can still eat the ones with black ends. Just cut it off and eat the rest of the tomato. What you're experiencing is very common. Ifyou start watering the same amount on a consistent schedule, you should see the problem go away.

Try adding some calcium to the soil with lime or crushed egg shells mixed with water. The lime may take awhile to act.
There are also calcium products that you can buy in Home Depot or a good garden center that may act faster. I always give my plants a boost with fish emulsion when they are suffering. It always seems to perk them up.

Even the best gardeners get blossom end rot when conditions are just right.

It's nice that the problem is an easy one to solve and not something that is incurable.

Penny
Zone 7b - North Carolina