Blossom end rot on tomato fruit is caused by insufficient calcium in the tomato tissue. It can be caused by:

• too little water uptake by the plant due to the actual lack of water in the substrate or the inability to uptake due to underdeveloped or damaged root system
• insufficient nutrition, including an actual lack of calcium in the substrate or the inability to uptake by the roots due to insufficient substrate moisture or damage to the plant root system
• excessive concentration of potassium, magnesium and ammonium nitrogen, and sometimes also sodium in the substrate (substrate salinity)
• excessive moisture, flooding the substrate or large fluctuations in the moisture content of the substrate,
• low air humidity (below 60%), especially during flowering and fruit setting,
• rapid fluctuations in air and substrate temperature.