I thought blossom end rot is always caused by soil conditions, viz. allowing the soil to dry so the plant can't absorb calcium. I get it every now and again and it goes away if I give the tomatoes enough water.
I've heard that it may be a shortage of Magnesium. That said since you apply Epsom Salts mixed in water, maybe the extra water is all they needed?
At least a little Epsom Salt does no harm, but it would be an interesting side by side test to do.
I have used calcium nitrate which can go in the water for the soil or be sprayed onto the tomato leaves. It appeared on BBC Gardener's World > 20 years ago. They did say the real problem was lack of water, so calcium or magnesium (isn't Epsom salts magnesium sulphate?) will both help, but it is really the H₂O you dissolve it in that matters.
When I plant my tomatoes I toss a scant handful of epsum salts into the hole. (Noticed that carrots that got an accidental dose did exceptionally well.) Later, as plants are flourishing I crush Tums and add a tablespoonful around the stems for calcium to prevent blossom end rot. Of course they get deep watering a couple of days a week. So far so good.
Each generation has its own rendezvous with the land... by choice or by default we will carve out a land legacy for our heirs. (Stewart Udall)
Cherokee Purple is the hands down, grow every year favorite here. The best flavor ever. I can, make sauce, spaghetti sauce, pizza sauce, salsa, and bar be cue sauce. I also dehydrate them. Fresh eating is awesome, they make mid size to large tomatoes, have green shoulders, sometimes are ugly, but get over it. Do you want exceptional taste or a pretty, red baseball? LOL
Diego Footer on Permaculture Based Homesteads - from the Eat Your Dirt Summit