Why Are Do M Roma Tomatoes Turning Brown on the Bottom.
Certified GKH Gardening Expert
Here is more information: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-blossom-rot.htm
I know your pain. The same thing happened to me last year. The fruit looked fine, but when you turned it over you would see these ugly brown/black spots on the bottom.
Based on what I found, it seems like it could be a calcium deficiency or the result of irregular watering. I also think it has something to do with too much water. I found that the plant that had the problem was in a wet area in my garden. The soil never seemed to dry up as well as the soil near my other tomato plants.
Have you tried adding crushed egg shells to the area under the plant? Maybe even some powdered milk?
Best of luck,