For me, this is a discovery! Thank you my Dear friend @Siggjo, for this wonderful lesson in botany and great photos. I did not associate these black spots with oil, or oil, I thought that, once upon a time, there was a layer of rotten algae at this level, which began to release hydrogen sulfide. Usually, such a layer of black color and it paints everything around, it looks like a very stinky oil, but it is not. Good night to you!