It is called BER (blossom end rot) and is common with the first early fruits thanks to the spring weather, more common in some varieties than others, and more common in roma-types than other varieties. It goes away. ;)

The FAQ below provides more details.

Dave

If you want to read more about it just do a search here using 'BER blossom end rot'. Lots of discussion on it.

Here is a link that might be useful: BER FAQ