Blossom end rot is due to a calcium imbalance in the plant. It can be brought on by hilgh heat, too much or too little water, over-pruning the tomato, too much nitrogen fertilizer and a simple lack of available nitrogen in the soil.
The following is from OSU.
A localized calcium deficiency due to any soil or growing condition that affects calcium uptake. This physiological problem is common, especially in home gardens. Blossom-end rot often occurs when soil moisture fluctuates. If too little calcium is in the soil or if the soil is high in salts, calcium uptake will be impeded, especially under periods of sudden drought stress. Excessive nitrogen applications, especially in the ammonium form, can increase a plant's demand for calcium. Once calcium is used in the plant, it becomes immobilized and cannot be translocated from older tissues to younger, growing tissues, which need calcium.
Symptoms A water-soaked, light tan spot at the blossom end or side of a fruit. The spot enlarges, becoming dark brown or black and leathery. Normally, spots are dry but may become soft if secondary bacteria and fungi invade the fruit. Affected fruit ripen faster than normal.
So what do I do about the rot? Will bone meal ie milk help? I don't think I over pruned. Barely did any this year. How do you tell if I over watered?
Harvest affected tomatoes and use as green tomatoes in recipes. There is a foliar calcium spray in garden stores which can burn if not used correctly. Since calcium is not very mobile in the plant, be sure it gets on the just forming fruits and also do the leaves. If you water and then check the soil, it is too dry if the moist layer is not below the root zone and too wet if it feels soppy. Wait 20 minutes after watering to check to allow fro drainage. I think the problem will be worse with the sudden onset of the hot spell, so even shading the plants might help as would mulching the surface. Good luck! Some varieties are more prone to this problem, usually the larger tomatoes. Just does not happen in cherry tomatoes that I know of.
Thank you. I also want to know about giving the plants milk and how much? I've heard that the plants absorb the calcium faster
I also heard of that but was informed that the calcium commercial spray worked better. If you use mild, use non-fat. At least it does not burn the plants. Let me know how it does if you doe that.