yes but they are still green when they are doing that...
so what do I do about it and how do I stop it?
You need to give the plants calcium. It is a deficiency that causes blossom end rot. A foliar feed will begin to help immediately. Anything you amend the soil with will take longer. Blossom end rot will also affect cucumbers and peppers.
Actually, more than hard. A few tomatoes have rotted on the vine . . . completely. Never had that happen before.
First . . . suggested remedies.
You can buy commercial sprays, such as Green Light's Blossom End Rot Spray, Bonide Rot Stop, and Ferti-lome Yield Booster. You might also visit your local drug store and buy epsom salt and apply it as a side dressing. Epsom salt is the treatment recommended by our local radio organic gardening guru.
Once it shows up I don't believe there's anything to be done about it. I just toss the tomatoes, but that's because I have so many plants I'm pretty much guaranteed to get all the good maters we can eat and more than enough to share with neighbors. If I didn't have so many plants I'd let the BER afflicted maters ripen, pick em, cut off the end, and enjoy! BER does not diminish the taste. Just looks bad.
My experience with BER pretty much is like what Carolyn Male, a retired professor of microbiology, describes in Blossom End Rot (BER) in Tomatoes
Both my father-in-law and daughter-in-law this season have grown BER-free tomatoes. They got their plants from seedlings I grew! We have similar soil, spaced them about the same, and gave them about the same amount of water at about the same time of day. Into each planting hole I included worm castings, manure/compost, and a time release fertilizer with calcium. Every few weeks I fed them Medina Hasta Gro Plant. So why was I cursed and they were blessed? Who knows?
Two seasons ago my father-in-law was the one cursing his luck with BER while I was BER-free. Last season we were both a little bothered with it.
I can tell you that my plants do provide lots of blemish free tomatoes, after tossing the BER afflicted fruit. My biggest headache now is staying ahead of the birds and squirrels! :-)