Cleaning wood with black stains

Trusses not a deck but, what do y’all think is causing the black discoloration? I’ve been led to believe it is a rust or iron stain. Or black tannin?

To simplify it, it’s not organic. Already tried several different SH ratios on it. And it’s not dirt, I used simple green and scrubbed it, I also used high pressure which removed a small amount of the black but most stayed the same.

Going to try a rust remover next weekend any thoughts? People have also mentioned oxalic acid?

Why start a new thread?

Other post didn’t get any foot traffic, also from a Reddit post I got more information, now I think it is iron stain but would like more opinions.

Your best bet is to call a professional.