Blossom end rot, maybe? Try adding a higher Calcium fertilizer (you can find this as a spray: http://www.gardeners.com/buy/tomato-rot-stop/33-447.html

help! Been growing tomatoes for years and occasionally got a black rotted tomato, but this year, that is all I'm getting.

Is it too much watering, too much heat,???? My other plants are all doing OK.....peppers, cucumber, etc....

TIA