Blossom-end rot begins with a small tan patch on the blossom end of a tomato (or pepper) when the fruit is still green. This patch turns leathery and brown or black as the fruit ripens and matures.
You can’t spray to get rid of it, because blossom-end rot is caused by calcium deficiency.
Moisture fluctuation and excessive fertilization can reduce a plant’s uptake of calcium. To prevent the condition, keep soil moisture even, use fertilizers high in superphosphate and low in nitrogen, and don’t cultivate deeply near the plant roots.
If your tomatoes are in containers, apply a tomato fertilizer that contains calcium. Luckily, you can still eat the part of the tomato that isn’t affected.