Blossom end rot caused by lack of uptake of calcium. This doesn’t necessarily mean you need to add more calcium to the soil though! It’s largely a watering issue. The plant is getting too much or not enough water and can’t make use of the calcium that may already be present in the soil. Tomatoes want about an inch of water a week and they are moody! Be consistent with your watering. Don’t over do it. Tomatoes will let you know when they feel neglected. There is a product called Rot Stop that is a calcium spray. Apply directly to the plant so it can uptake the calcium it’s not getting naturally.