What about when your water is liquid rock? GH of over 300, KH around 200. Is the calcium in hard water not an available form? I've never considered the fact that it may not be bio available. I don't usually have end rot on most varieties of fruit. Sometimes though.
I don't claim to be an expert; I'm just reading these articles on the benefits of using eggshells to increase calcium for my plants. I suspect that calcium in hard water is bioavailable to the plants. However, in eggshells, that calcium is tied up in a calcium carbonate bond that needs to break before the plants can feed off that calcium from the eggshells.

Since then, I have moved my gardening to my backyard and have access to my house water for a sprinkler system. Also, I got more into making chicken run compost and I think that has the improved my soil in the raised beds more than anything. I now use the hügelkultur method in my raised beds and I think the large water log batteries in the base of the raised beds helps to more consistently feed my plants. Since I have made all those changes, I rarely see blossom end rot anymore, although not 100% eliminated, I would say maybe 95%, or more, free of blossom end rot.
I am constantly adding sulphur and iron to get the pH of my soil down below 8.... It's a nightmare and a half but I have managed to keep ph near 7 with compost, sulphur and peat. I use the epsoma soil acidifier on my berries every fall.
I use lots of chicken run compost in my raised beds. Compost is supposed to have a pH around 7.0, give or take. I have not checked the pH of my beds. However, my plants are growing tall and strong, producing lots of food. If I had a problem, I would probably check the soil pH in each bed. But I'm just a hack gardener, doing what seems to work for me. None of my plants require special pH levels, so I just leave them alone and they seem to grow well.