Thank you for including an image with your question. Your tomatoes have blossom end rot (BER).  It’s a calcium deficiency but only because of uneven soil moisture. When the soil or potting mix dries, the calcium isn’t transported to the bottom of the fruits. 

BER is most common early in the season, when the root system is still on the small side.  It’s also rather common with container-grown plants; in this case, the roots have a limited area from which to absorb water. The third instance is with certain paste tomatoes. BER has been a common complaint during this year's unprecedented heat wave. Soil (or potting mix) is drying out much more rapidly than gardeners expect.

The remedy is making certain the plants have an even supply of moisture at their root tips.  Not soggy, but evenly moist. No need for special mixtures in the planting hole. Calcium sprays don’t help either.