Black rain is just like rain but it fills the water butts with contaminated water, and infects other, pure water with contamination. Black rain happens when a player survives 15 days. By drinking contaminated water, you risk your character getting radiation poisoning. You can still use toilets, showers, objects which consume water and even go on expeditions without getting Radiation Poisoning. When the black rain is over, water starts de-contaminating. Then you can drink water without risking your characters. These are the risks of drinking contaminated water:
No chance getting sick: 0% - 10% contamination
Low chance getting sick: 11% - 25% contamination
Medium chance getting sick: 26% - 39% contamination
High chance getting sick: 40% - 74% contamination
Confirmed sickness: 75% or more contamination
When you go out without a hazmat suit while the black rain is happening, you get Radiation Sickness quicker, making your character die faster. And when you go on expeditions while it's raining, it slows the whole expedition, decreases stats slightly faster. It is recommended to not go to expeditions during black rain. When the black rain starts, maintain all characters with water ASAP (as soon as possible). That makes the characters not thirsty during black rain. If the contamination is more than 10%, it is not recommended to do this, or you'll get your character into Radiation Sickness. To cure the sickness, you have to use Anti-Rads (anti-radiation tablets) and the sickness will be stopped. If you don't have any of these, you'll have to go to an expedition, and go to pharmacies or places like that. If you don't find any, you can make makeshift Anti-Rads. If you cannot do this, it is time to say goodbye to your character. Always do not drink water when the contamination is more than 10%!