I know it's an odd question but very curious if others have seen similar issues.

I started my Cherokee Purples this season in hydroponics on 11-28-2020. On Jan 1 I put them out into the garden along with several other varieties that I started in pots. All 16 plants are now over 5' tall and producing like nuts but each and every Cherokee has BER. I even culled all fruit off of one and let it set fruit again to have the same results.

My very first year with BER on anything. The only thing different between all the garden is they were started in Hydro.

Makes perfect sense to me, since the only time I have ever had trouble with BER is when I could not keep the moisture levels consistent. Extremes of moisture between wet and dry for me seemed to bring it on and there was no going back. For me it was trying to grow in pots during a hot summer. No way I could water in the heat because I was at work all day. I would water in the AM and when I got back in the evening but the pots would be dry by then. Not sure I had any usable fruit from the large plants in those pots. I can't think of much more extreme in the moisture department than going from hydroponics to soil. Maybe it would be possible if the soil was kept very wet, soupy, and very slowly dried out, but I don't know. Maybe some with experience with hydroponics will be able to help you more.

Another thing to consider is, if you have your CP together then maybe there is something in that part of the soil? Run off, metal, lime, etc.

If it's a new spot, did you get a soil test? If the BER is only on the one variety, I'd order some seeds from elsewhere. The only times I grew Cherokee Purple, they had a little early BER, but outgrew it quickly, which obviously yours haven't done. For now, it might be worth trying a calcium spray, after pulling all of the infected tomatoes.

But, as Sue noted, it's mostly watering irregularities seem to cause blossom end rot.
And if none of your others are having the problem, and it hasn't happened before, you obviously aren't doing this wrong. So a problem with the cultivar seems more likely. I've even had some varieties do this in SIPs, and you can't get more even watering than that! But were one timers, not varieties I'd been growing for years.

A little more info...
All of my Cherokee Purples were started in Hydro and transplanted into the garden soil within a foot of the other varieties. I do indeterminate only and trellis / trim well. Early on I trimmed all plants into 4 main stems which I can manage in the fairly small space we grow in. I took several large suckers from the main plants to root and gave many away as well as used several to complete the garden. Each and every one of those cuttings have the same BER but are massively productive. No other varieties have BER growing in the same soil. I've one with 26 fruits on a single truss.
I'm now leaning toward it being a bad seed to begin with and saved seeds for this falls garden just to see if they grow the same. They have the great flavor still.

All of your other plants were also started in hydro? It definitely sounds like some bad seed, though I don't know what would do that. I have had other varieties that had BER on every fruit, but only grew them once - maybe they had a similar bad gene that got in them! I just never tried them again.

Maybe try one or two from the same seed batch, started in soil, and one or two in hydro again, and see if they both get BER? If not, maybe it is the hydro that did it with them; if both, then it's the seeds.

I am still confused about what you did. You are comparing the Cherokee Purple plants to plants of other varieties in the same garden and soil, but were those also started hydroponically and transplanted to soil, and have no BER? Or were only the CP plants started hydroponically before being transplanted to the soil? Also where are you getting the information that the BER might be caused by bad seed? I have never heard of that before, always that it was an environmental issue.

It's probably just a coincidence that they were started in hydro. Plants that grow very vigorously and fruit expanding fast are more exposed to ber, in case your watering is fine. I think you might need calcium supplements, especially if you grow in amended beds with little actual soil, like it's popular now. And cutting ber affected fruit will make the remaining fruit expand faster, and again be more prone to ber.

Pythium root rot can also cause ber, when the ground is too low and wet. Early rooted hydro transplants should do fine, but if they have a lot of roots, they can clump together and rot, which could theoretically display as ber. I dont know that this is what is going on for you, just food for thought.