Sounds like a calcium deficiency. Soil's ph should be 6.0 to 6.5.....you would have to do a test..... Next year you will have to add lime to your soil, mulch & fertilizer....we also put that black garden plastic down to keep the plants moisture even there is a calcium solution that you can spray on the plants be careful with calcium chloride can spray if temsp are over 85 as it can burn the plants. follow directions....they still need proper water. It is suggested that you try a different variety of tomatoes that are grown to prevent end rot. You can remove the fruit with end rot so other blossoms grow faster and produce more fruit. If you leave the damage fruit it will not regrow or repair itself and it could serve as an entry point for disease bacteria, fungi and insects. The source for this information from the Alabama Coop www.aces.edu/dept/extcomm/speciality/ber.html for full prevention information some gardeners have ground up egg shells that they have added around their plants...you also could add pinestraw, straw, decomposed sawdust, plastic or newspapers. This conservice soil moisture and reduce incidence of blossum-end rot.