Q.Blossom End Rot on Plum Tomatoes
We planted several varieties of tomato plants. Our grape tomatoes are doing very well, yielding lots of large red tomatoes. In the same garden we have plum tomato plants and they have started to ripen this week, but we have a problem. The bottoms are black and insides look fuzzy and moldy and are not edible. My question is: how did the plants get this disease, and will all the fruit be spoiled by it on these particular plants? Is there something we can do?

Certified GKH Gardening Expert
Blossom end rot is treatable and is normally temporary. This article will explain why it happens and how to fix it:
https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm