Have you done some test spots in many different areas on the very black stuff?
That looks like split face block which is very porous and holds and absorbs the stain easily.
Before going too far, go do some test spots to see if it will come clean.
I was working on some split face block and after trying around 25-35 different chemicals and combinations, there was nothing that would clean it (maybe HF Acid but it would have to be very strong and probably unsafe and cost a lot) so we gave up. We called and talked to many of the suppliers out there, The chemical companies like Prosoco, EacoChem, etc.... and could not get it cleaned affordably.
It was block that was stained so it had a "checkerboard" look when you stand back and look at it.
You don't want to run into that kind of situation.
Just a FYI. Good luck.
_________________
Superior Power Washing<O</O
Chris Chappell<O</O
361-853-2513<O</O
prostaff@superiorpowerwashing.com<O</O
Exterior House Cleaning Corpus Christi Texas<O</O
Cleaning Driveways and Sidewalks Corpus Christi Texas