The patio
I recently cleaned a patio in Marlow where this had happened quite noticeably across the entire surface.
The black spotting you see on patios like this is usually a type of lichen. Unlike general dirt or algae, it embeds itself into the surface of the stone, so it won't simply wash away with a hose or a light clean.
The result
The clean made a significant improvement to the colour and overall appearance of the patio.
Worth being honest about: some heavily embedded spots can be more stubborn, and the deepest black spot doesn't always lift completely in a single clean. Even so, the difference after a proper clean is usually very noticeable.
I've covered what causes black spot and how treatment works in more detail in a separate guide on this site.
