I noticed that my sink drain is covered with some black substance. Same thing covers the stopper. I remove the stopper, clean it; it stays clean for several day, than this substance grows up again. Any idea what is that and how to fight it?

Thanks.

Make sure you are using plenty of water to flush drain and trap when using. Are you getting black residue elsewhere?

Don't see it in the bath tub or in the shower, but It looks like drain in the kitchen sink also has it. Do you know what is that and if that's dangerous or just annoying?

Thank you.

Anything from a septic system can be dangerous. Drains are part of a septic system. Is the water level under the sink drain very high? Is the trap very close to the bottom of the sink?
It also could be a sluggisk drain and the residue inside the drain simply rises as it fills to drain. Is the sink drain sluggish? To test, a drain should take water away faster than you can fill it. Fill it about 2 inches and let the water stay on, then release the drain. It should completely drain the sink as you are continuing to fill.