looks like someone dumped a charcoal grill there

It must be some type of fungus to keep coming back. You could contact a horticulturist, possibly at a local university for advice. You could also Google "black fungus on rocks", for identification and treatment.

Do you have a sprinkler that hits that area - or some other water collecting there?

It's really common around here(zone 9b) for groundwater/well water to leave orange rust stains like that - I've seen it often on the sides of buildings and pavement - from sprinklers, but also from air conditioners and rain gutters.

There's also a red algae that forms in damp spots...