It may be blossom-end rot, which sometimes doesn't appear on the outside. In this form, it's sometimes called "black heart".

Yeah, it's a terrible disappointment. Below is a reference to it:

Here is a link that might be useful: Nice outside/Black on the inside

Not many people know it, but blossom-end rot can create black spots inside the fruit, with no external symptoms. It's rare, but it happens sometimes.

The solution is the same as it would be with normal BER: even out the moisture levels, correct overly acid soil, and wait for it to go away.

There's a FAQ on BER in the main Growing Tomatoes forum.

--Alison