Look at the BER or Bottom End Rot section of the FAQ... Too much water, not enough calcium, or a problem with the soil are common causes... Good luck...

Probably Blossom End Rot. Caused by Calcium not getting to where it helps. Evenly watering usually remedies this. Just toss the bad ones and the rest should be OK. Happens alot on the first fruits of the season. Most any stress to the plant will trigger it like very hot weather, way too much rain, root damage, etc.

Excess nitrogen fertilizer is also a contributing factor. Go easy on the MG. ;)