Have you checked out this article?I'm growing one Roma tomato in a pot (the rest in GrowBoxes) that has had blossom rot from the get-go. But the tomatoes are also ripening too quickly--they're the shape of Romas, but the size of grape tomatoes. For the rot, I've tried a "miracle" spray a friend gave me (no label but directions: spray blossoms, three times, five to seven days between), bone meal, moving the plant out of partial shade. None of it has worked. For the ripening problem, I have no clue.