Tomato fruits have a black sunken patch at the bottom end, is this a calcium problem?

A dark, sunken, leathery patch at the bottom blossom end of tomato fruit is blossom end rot, which is linked to calcium deficiency in the fruit. The important point is that it is usually not because your soil is low in calcium, but because calcium did not reach the fruit. Calcium moves with water in the plant, so uneven watering, letting the soil dry out then flooding, high salinity, root damage, or very fast growth all block calcium reaching the fast-growing fruit even when the soil has enough. That is why the leaves can look fine while the fruit rots. Please confirm by checking your watering pattern and a soil test before reaching for products. The most effective fix is steady, even soil moisture, often with mulching to buffer the soil, plus avoiding excess nitrogen which pushes too-fast growth. If the soil is genuinely low in calcium or acidic, liming or gypsum before the season helps, and a foliar calcium nitrate spray on the developing fruit trusses can reduce new rot, though it will not cure fruit already affected. Adjust to your soil test and local KVK advice and do not over-apply.