Acid. Not for the faint hearted. It would help to know what the black is. Algae? Iron staining?

Try using a hull cleaner that you can purchase from a large boat store or marina. It is an acid but not as severe as some.

Hi, Mantine - You really need to know what minerals compose those (formerly) white rocks. Most any acid could do a really destructive job on them. Take a handful of the rocks to your local nursery and ask them what to do - please don't just go pouring stuff on them to see what works! Good luck.

I would get rid of the white rocks. Haul them away or bury them. The glare alone is reason enough for me to not use them. As you found out, they don't look white after a few years and are next to impossible to clean. Pouring harsh chemicals on them can't be good for the soil or plants.

I saw a thread on examples of bad landscaping. You'd be surprised how many of them had white gravel.