Without having more specifics, this problem could possibly be blossom end rot. This is when a small water-soaked spot appears near the blossom end of the tomato. As it enlarges, the spot becomes dark brown to black, sunken and leathery (i.e. it rots). This happens when calcium is not readily available to developing tomato. Calcium imbalance can result from fluctuations in soil moisture caused by improper watering or prolonged dry weather. Other causes are high nitrogen levels from fertilizer, or a disruption of the root system. You can prevent blossom end rot by the correct application of nitrogen (from fertilizer), and keeping the plants mulched to maintain moisture. Mulching also helps to control weeds and eliminate the need for cultivation that can damage roots. For more information on all your tomato gardening problems visit Denver County Extension Program. In the meantime check out our Top 10 Ways to Enjoy Tomatoes for those tomatoes you have already grown.