Some people said it's because , lack of calcium , so how to solve this problem naturally , if I don't want to buy chemical products that costs a lot in Pakistan so how can I get a cheap solution ?

The tomato plants appear healthy, but as the tomatoes ripen, an ugly black patch appears on the bottoms. The black spots on tomatoes look leathery. When you try to cut off the patch to eat the tomato, the fruit inside looks mealy.