Your poor little tomato had "blossom end rot", the cause of which is vigorously debated by various gardeners. I have learned to sprinkle a small amount of garden lime (sold in hardware stores, garden centers) about 6 inches out from the stem of the plant, on top of the soil. If it gets too close to the stem, it will burn the stem. This way, every time you water, or when it rains, more lime is going into the soil. I've been doing this for over 10 years now, and have completely eliminated blossom end rot in my tomatoes. There is also a spray you can buy, but the lime is less expensive. Oh, and BTW, can't overemphasize the importance of MULCH, even just a small amount of hay, straw, or wet newspapers on top the soil. Or, red plastic, which has been shown to improve tomato yields.

I garden organically, and I know this is a long post, but I get carried away by my passion for gardening.