I'm a former WSU Master Gardener of 9 years and a commercial U-Pick gardener of 40 years and I have had experience with this problem, too.
This one is easy. It's called Blossom End Rot. It is not a desease, but, a deficiency.
Just add some calcium in the form of bone meal into the soil and water it in. That should do it.
Next year plant your tomatoes in a different location and put a handful of bone meal in the bottom of the hole. And plant varieties resistant to Blossum End Rot. I'm partial to hybreds because they have often been bred resistant to a lot of conditions and deseases.
The Tomato Lady