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Before I can try to research your question, I need more information.
What is the variety? And is this ripe stage green? 

Thanks for answering, after researching your question. I am still unsure of an appropriate answer. 
Big girl is not typically a cherry tomato. and frequently as a tomato ripens it is a bit darker at the top.( but not black)  If they are still in the green stage I would wait to see how it develops.

There are many varieties of green,  purple and black cherry tomatoes. Perhaps you purchased one of these varieties?  Again,  I would wait to see  how it ripens. If you bought the seeds from a seed catalog, you might contact them to ask if this is " normal" for the variety you have.

I grow a black krim that is a darker green on the stem side ( top)  in the early stages and as it ripens gets more purple on top and reddish bottom. It never fully looks ripe, but surprisingly it is ready in this stage.
Have you tasted the tomato? does it taste ripened?

I could not find any scientific reasons for the discoloration. but this intense heat and drought can effect many ways a tomato ripens.

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