Two kinds of black on the same floor
The first kind sits on the surface. It is a mixture of carbon dust shed from stacked tyres, rubber picked up by trolley and forklift wheels, and general grime, and it responds well to a wet method with mechanical action. Removing it makes a noticeable difference and it is squarely a cleaning task with a predictable outcome. Sites usually notice the difference within a single attendance, and that first result gives everyone a sensible baseline for judging the visits that follow.
The second kind has transferred into the slab. Where a floor is bare, worn or lightly sealed, rubber works into the surface under load and stops behaving like soil at all. Repeated aggressive attempts to lift it use time and product without improving the result, and on a worn slab they can open the surface further. Recognising the difference is what keeps a scope honest.
