Disappointed. Not the product advertised. I don't know how well this works yet as we've not tried it (brought it for a child's birthday gift amongst a pile of pranks) but it's a cheaper version of the product advertised. The 'soap' doesn't look like soap - you can see the black grains - and the packaging is slightly damaged.
Unless I received a faulty product, this soap certainly doesn't produce any black marks. I'm not sure of it's antibacterial properties, so can't rate it on it's function as a soap, but as a practical joke it fails.