Probably called blossom end rot.Use epsom salt to prevent this. I put this in the soil when I plant them. Try putting some on top of the soil and maybe the new tomatoes won't get. Epsom salt is good for all plants.

Thanks that's what I thought at first but it's slightly different. It actually goes right through the fruit. Does blossom end rot show up inside the fruit? I've never experienced this before in many years of growing. I will try the Epson salts and see if that helps.

Looks like this?

Dept. of Plant Pathology Cornell University has a very readable discussion of causes and remedies at Blossom End Rot of Tomato

Edit to add: Check out the "control" section here: IPM : Reports on Plant Diseases : Blossom-End Rot of Tomato

Epsom salts can make the situation worse. If you read anything about BER at all, you'll see that it is caused by the plant's inability to access calcium in the soil. This can be caused by a few things: a very wacky soil pH, uneven soil moisture levels, and an excess of other fertilizer salts such as magnesium.

Adding epsom salts (magnesium sulfate) interferes with the calcium in the soil, and should never be added without knowing for certain that your soil is extremely deficient in magnesium, a rare occurrence. Plants have a need for magnesium and sulfur, but in very small amounts already available in a healthy soil system.

A very high OR low pH will also lock up the calcium to soil particles, making it unavailable to the hungry roots of the tomato plants.

Finally, soil moisture must be deep and regular for the calcium ions to remain in the soil solution. If the soil dries out in between rain events, the calcium becomes locked up again.

If you are growing your tomatoes in actual soil in an actual garden, a professional soil test is important. I don't trust most of the kits available to gardeners, unless tested against the results from a lab.

A good quality soil-less potting medium has already been pH adjusted, allowing you to add gypsum, a calcium sulfate mineral that will not raise the pH while adding calcium. Most calcium fertilizers will raise the pH, something you only want to happen if it is necessary.

The practice of adding epsom salts has long been debunked as one of the many gardening myths handed down from person to person. In the case of tomatoes (among others), it can cause problems. Don't ever add it as a cure.

Indeed "The practice of adding epsom salts has long been debunked ...."

On the other hand "...gardening myths handed down from person to person..." is still a very popular nuisance.