It is Blossom End Rot and there are many theories as to why this happens. Some are convinced it is improper watering. Others say it is a lack of calcium in the soil. Others like myself say it is because the plant is unable to uptake the calcium. I do know and many others know for a certified fact that in their soil there is a 100% cure against this disease, or whatever it is. This "cure" is using epsom salts. I use a handful when I plant the transplant and another handful when some of the first fruits are about 1/2 size. If I don't do this I will still grow a lot of tomatoes but every single one of them will have BER. Many others on the forum must also use epsom salts. Some don't use as much as I do and I am sure some use more. I use it on everything in my garden with the exception of corn and okra. It will not help a tomato that shows the rot, but it will stop further fruits from contacting BER in most cases. I have advised this approach for years on this forum and not a single person so far has posted back that it did not work except for one poster who only used a couple of tablespoons per gallon of water. I have fairly big hands and I estimate that a handful of ES is about 1/4 cup. I don't think it is excessive but others do. I just know it works. Try it. You might like it. What have you got to lose?