Strange black stain on concrete wall

So I have been cleaning for 5 years now, but never had this problem. It’s a brick building (1907) with a false block finish. A concrete like facing made to look like stone and mortar. Anyways, the 4 story building has an exterior boardwalk going up the side, and the black stain is underneath this walk. the rest of the structure cleaned up fine, but I am lost as to finishing this one section, which was the primary concern of my client. Any thoughts?? FYI I have tried undiluted Liquid Shock (12.5%) NMD80, One Restorer, HD Britenol, F9 Eagle, and Turbo Nozzle, without any luck.



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I love your safety cones! seriously, not tongue in cheek or anything, great way to use things. If my empty 55 gallon drums weren’t so big I would use those.

Just a hunch, looks like an oxide of metals to me…obviously not organic if SH did nothing.

Post this on the F9 Facebook board, I’m sure there’s a way to get it off.

I have 4 traffic cones, but we had a terrible time trying to keep people and vehicles out of the way. So we lined up these 2.5 gallon SH totes to make it obvious. The dumb thing is: people still weren’t getting the point. UGH!

Run some hazmat caution tape around those cones.