I've been growing the Black Krim variety for a number of years. Oddly enough this year whilst the fruit has ripened, the top part which should be a deep purple/black has remained resolutely green. Every one the same both in GH and outside. Perfectly edible - nice and juicy.
No mystery, it's called 'Green Shoulders', common with large varieties like Black Krim. Usually caused by too much heat or direct sunshine, often because people have removed too many leaves due to misunderstandings about the ripening process.
Mine weren’t ripening at all, despite the usual tricks. So, to make sure I had some before the weather turns I went extreme. I have pruned off ALL the leaves, cut the top to stop them growing any taller and also cut off any really tiny ones or flower buds that have no chance of becoming full size, ripe tomatoes in time (basically, what Monty said to do in recent GW episode but maybe going a bit further with the pruning). Mine have now started ripening and I have picked 2 or 3 off within only a few days of taking these actions with (what seems like) more turning orange or red, each day since.
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Usually caused by too much heat or direct sunshine, often because people have removed too many leaves due to misunderstandings about the ripening process.