Well I could talk about this for days!
Blossom End Rot is caused by a calcium deficiency at the fruiting area. That may or may not indicate a calcium deficiency in the soil.
The best way to treat blossom end rot is to prevent it. A soil test will tell you if your soil needs amendments, and if you need calcium, you should add lime months before planting. Your Extension Service will do it, or refer you to a testing company. Costs about $20. Well worth the price.
If you have blossom end rot now, here are some possible causes, and potential remedies:
1. Overwatering or irregular watering. Tomatoes need a good deep watering, on the ground, not overhead, about twice a week. If you don’t get enough rain, about an inch a week is enough. Tomatoes benefit from a little “drying out,” and folks tend to overwater out of sympathy. You are hot and dry, so your tomatoes must be, too. I always stick my index finger into the ground. If I feel any moisture, I wait another day. Who knows, it might rain.
Nutrients travel to the different parts of the plant via water, so irregular or overwatering can prevent the right amount of calcium from reaching its goal, even if your soil is perfect.
2. Too much magnesium. That is why adding Epsom salts may not help, and may make the condition worse. Magnesium feeds the foliage. Fast foliar growth can divert nutrients from the fruiting area. Again, a soil test can tell you if you have a magnesium deficiency, but unless you are container gardening, or farming intensely, it is unlikely.
3. Fast foliar growth can happen without adding Epsom Salts, so many times, the first tomatoes have BER, but the condition will correct itself in a week or so.
4. Finally, insufficient calcium in the soil. Very few soil additives are going to help at this point. Remember, I said add lime months before planting. Probably crushed Tums or chalk will work most quickly. Maybe watering with a diluted milk mixture. It is unlikely egg shells will work, but if you try them, put them in the blender first.
There is no consensus on foliar sprays. I use one when I have a few bad tomatoes. I feel like it works, but it is just as likely that the condition corrected itself. There are organic foliar sprays. Green Light makes one.
Don’t panic when you see that ugly blotch on you precious tomato. Think through these steps, correct where necessary, and your crop should be fine.