The black stuff when you rub your hands together is a combination of dead skin and natural oils.
When you rub your hands together you mix those dead skin cells and natural oils together which causes the blakc stuff.
The black stuff when you rub your hands together is normal and it's basically just dead skin cells that get mixed with your natural oils and what ever else sticks to your skin throughout the day.
Some people make more skin cells and oils than other people.
You need to “exfoaliate" or wash this stuff off but it takes a while for water to soften and loosen everything up.
When you rub your hands together, the action of scraping the surface of your skin back and forth against each other causes the molecules in your skin to move a little faster.
The faster that molecules move, the higher the temperature.
So the friction of rubbing your hands together makes them feel warmer.
It is not bad to scrub dead skin off.
It is actually very beneficial and even something that you should do quite often, but you should do it properly.
For example, all those DIY scrubs “recipes” you find online do more damage than good.