This sounds like a disease called "blossom end rot" and is caused by either lack of calcium in your soil or erratic watering. Tomatoes need consistent water but not a lot of water. Feel the soil down to one knuckle on your bare finger. Water when it is dry not on a basis of every day or once a week. You must feel the soil. As for calcium, use a plant food with micronutrients such as Osmocote Flower and Vegetable that contains calcium as an additive. The condition will not spread to other plants and your crop should be fine next year if you consider your watering and your nutrients. Keep growing, Marianne Binetti