Cat rubbing on almost everything!?

My female cat Mittens has been doing this since we got her. She rubs her mouth on my hands and other people hands, she rubs it on any sharp things she can find. Why though, I don't understand? A couple of weeks after we got her she realised the cupboard with the cat treats in which is in my bedroom. After that she started to rub she mouth or chin on the corner and after a while it ended up with this black stuff with her fur mixed in with it? what is it and why dose she do it?