Black spots on bluestone/flagstone don't wanna budge with SH

So as some of you may know on bluestone you get those black mold spots that don’t budge. i can SH at 1 % or 10% and the top layer turns white then you gotta wash it off , let it dry and do that a million times and sometimes 10% or 12.5 doesn’t do anything or high pressure. so what is next?

I do high pressure high heat first, but then you literally have to apply pure undiluted 12.5% sh, let dwell until it dries, sometimes brush in as well. Once it dries 95 to 100% of them will be gone