Bluestone Spotting

Hey everyone, very new to the pressure washing business. Only just started everything up this season and with only a few jobs under my belt at this point. I did my dad’s house for him and came across an issue when cleaning the bluestone. Applied 1.5% SH and let it dwell a few minutes before going over it with the surface cleaner and rinsing after. Here is a picture from a couple days later. The picture I included reminds me of a water damage look. It seems to be surrounding the joints. There are also leaves on it but if you can see past the leaves, you can also notice some spotting.

Can anyone help me figure out:

A) What happened or why it happened.

B) A better way to perform this type of work going forward.

C) How to fix this (if not permanently damaged)

Any insight would be greatly appreciated. Thanks

Looks like dirty water settled and dried there. Maybe you didn’t rinse well enough. Put some 6% on a spot and see if it goes away. The black spots could be tannin stains. 1.5% and pressure often won’t do it. Again, a stronger mix and they should disappear.