It's called blossom end rot. You may be able to salvage the top of the tomato if the rot hasn't spread. It's a physiologic conditions caused by lack of calcium. It may not necessarily mean that your soil lacks calcium, but that your tomatoes are not taking enough in due to irregular watering or lack of magnesium. Epsom salts are sometimes recommended as a preventive measure. They can be watered into the soil.

Here is a link that might be useful: blossom end rot

Epsom salt won't help.

The problem is that the plant isn't getting the calcium all the way to the end of the fruit.

The problem is erratic or uneven watering. Soil must be consistently moist.

The FAQ on this forum has the explanation
http://faq.gardenweb.com/faq/tomato/

Here is a link that might be useful: BER on FAQ

Epsom Salts ; I mix a Tablespoon to a gallon of water

Roma types are especially prone to BER.

Occasionally you'll find BER inside the tomato with no sign of a problem on the outside.

While many internet sites state that BER is caused by calcium deficiency and watering/moisture stress, the BER itself seems to be directly caused by the undetached blossom from the fruit. Has anyone noticed that the dried blossoms of tomato flowers are usually attached on the tomatoes with BER, thereby causing moisture to be present at all times at the bottom of the fruit? I think the same reasoning applies to catfacing. The undetached flower blossom is not allowing the bottom part of the tomato to be cleanly close, as if human's navel would still have the umbilical chord would cause a problem if it did not dry well and detach itself. So I have been working on taking off the dried flower blossoms off the bottoms of my young tomatoes.

Possibly, calcium may aid in drying and detaching of the done blossoms, but in the meantime, I think removing and aiding the tomatoes to have a clean bottom may help.

Has anyone noticed that the dried blossoms of tomato flowers are usually attached on the tomatoes with BER?

No.

This is a problem with a known cause. See the FAQ, What is blossom-end rot? How can I prevent it?

"I think the same reasoning applies to catfacing."

NO!

the catfacing is pollination during cooler temps.