Hi- this appears to be blossom end rot. This is a common nutritional disorder of tomatoes caused by a shortage of calcium in developing fruits. Fruits may experience a calcium shortage if there is a calcium deficiency in the soil or, more typically, if plants are subject to extreme water fluctuations. Factors that encourage blossom-end rot include: low soil pH, low levels of calcium, inconsistent watering, shallow watering, droughty conditions, and excessive use of nitrogen fertilizers. When symptoms appear, remove fruits immediately. Plants usually grow out of this condition as the season passes. To prevent the problem maintain soil pH in the 6.3-6.8 range, mix in a handful of gypsum (calcium sulfate) or ground limestone into the planting hole prior to transplanting, keep plants well mulched and watered through the growing season, and avoid overfertilization with high nitrogen fertilizers.
The problem should self-correct if plants are being watered regularly and evenly. If plants are receiving regular, deep watering during dry weather periods and you continue to observe the issue on newly formed fruits you can spray the plants with commercially available products (ex: "Stop Rot"), that contain calcium chloride.
See our website for photos and more information: http://extension.umd.edu/growit/blossom-end-rot-vegetables