I lost the top to a tomato plant in a weeding accident with a hedge trimmer,I let the plants sprawl and it got mixed in with weeds.
It had 2 good sized green tomatoes on it.
I put the severed part in a juicy potting soil mix in the hopes the cutting would root.
One tomato started to blush 1/2 and 1/2 but had BER.
The green tomato is still fine.
So in my case it has nothing to do with calcium,it's like your toes not getting blood and turning black.
The cutting is still alive but distressed.

Unless I'm reading this wrong, your cutting still has a tomato or 2 on it? It will do much better if you remove the fruit, so it can concentrate on growing roots. Right now it's trying to sustain the fruit, and to ripen it eventually.

I didn't want to lose 2 fine tomatoes.
they were too green to ripen off plant.

slugworth wrote: ↑Wed Aug 04, 2021 3:25 pm
I put the severed part in a juicy potting soil mix in the hopes the cutting would root.
One tomato started to blush 1/2 and 1/2 but had BER.
The green tomato is still fine.
So in my case it has nothing to do with calcium,it's like your toes not getting blood and turning black.
The cutting is still alive but distressed.

I think it could be argued the calcium has to be moved "by the blood"...while the root system was severely compromised, the tomato did not get enough calcium uptake. You know, for healthy bones, and stuff. I agree though, I think blossom end rot, or the appearance of such, has more causes than "soil low in calcium". I noticed a ripe looking estiva F1 yesterday, up on the 4th truss, and I haven't even picked from the first truss yet. Pulled it off, the bottom was rotten and up into the tomato. I saw it blushed a few days ago and knew it was "wrong", but didn't get around to picking it. Maybe could have salvaged the top of it at that earlier date.

Another green tomato on the severed limb is still fine.
The cutting is starting to develop roots.
I have seen BER on green tomatoes,but not for this episode.
People pick fruit at 1st blush and it ripens without developing BER.
Makes me wonder.