I think this is called 'blossom end rot', my first ripening tomato this season had the same thing. I read that this can happen early in the season, to just pick it off and hopefully the ones ripening later in the season will be fine. Possibly it's caused by a poor supply of water and calcium in the developing tomatoes. I think someone posted elswhere to add epsom salt to the soil before planting (our tip for next year!) Also tomatoes that were planted out before the soil warmed up are susceptible to blossom end rot because their root systems don't develop well in cold soil, so the plants cannot take up enough moisture and calcium for the fruit to develop.
I'm going to try mulching the bed now that the soil has warmed up - this should help to maintain even
soil moisture, instead of drying up constantly (mine are container plantings...)
