Black stain on concrete block

[dcbrock]

I’m thinking turbo nozzle?

[florida_condo_cleani]

I think 50/50 mix and a medium pressure wash.
I’ve never needed high pressure on that type of block. Let alone a turbo nozzle.

[Racer]

Exactly as Florida said. They clean up easily. I use my roof mix at about 3.5-4%. Comes right off with light to medium pressure
Use search bar. This has been talked about before.