Blossom end rot occurs when the soil with your tomato box becomes very dry then very wet. The contrast in the soil causes a calcium deficiency, this is seen by black spots on the bottom of your tomatoes. You can add calcium into the water reservoir which will help the rot. You can also add crushed up “Tums’ which will add calcium as well.
To avoid this simply use the Harvest to Home Watering system or make sure to keep your box full with water.