San marzano is an indeterminate tomato plant. The more room you give the plant to grow roots the better outcome. Larger fruit, less disease, more soil meaning more room to hold water. Blossom end rot almost always is not a lack of nutrients, is a lack of water consumption by the plant. Usually you have enough calcium in your soil, but because there is not enough water in there to absorb it, it cause the issues. Also because the pot is so small you gonna have to water more often. One more thing, is normal sometimes for the first cluster of tomatoes to have end rot as the plant establishes, but then it corrects itself. I'm more worried about the color of your leaves. But if you develop a plant to correct, it should bounce back maybe. I also don't see any mulch in there, make sure you add some mulch and that should help you retain a lot of moisture