Fred, I'm not sure how much help we can offer "remotely" and when you're not the person whose been caring for your pool...so take anything I or others say with a grain of salt
To me, your stairs look much more uniformly discolored than what I typically see with either stains or algae.
At the same time, your chemistry does not follow the TFP method so your water is "under- or - not- sanitized" by our standard. Going forward, please consider getting yourself a FAS/DPD kit like the TFP 100 or Taylor K2006 (
http://www.tftestkits.net ) and reading up on the [fc/cya][/FC/cya] and working through pool school.
If you want to get an idea whether or not it may be organic, you might set a puck on the step for a bit to see if it lightens the color. Then maybe work a small area with a wire brush. After that, you can try a crushed up vitamin c to note if the black reacts to that in any way. If it started out as copper staining, the black might deepen, then turn rust color and lift. Other stain sources include maganese, which does stain black/purplish. I'm surprised they did not test for metals. I would ask them to.
I have additional questions, some that you may not be able to answer:
- Is you're water source well or city?
- is your pool properly bonded?
- what Jacks treatment did the pool co. use and what did their stain test show?
- how long after the job did the black start?
- why is your salt level so high? What swg are you using -- please post equipment info in your signature
- do you have records of readings from each pool service reading from when it was done to when it started blackening?
You are in Illinois, correct? Or does Il stand for something else? Black algae is not particularly common in these parts. Is the pool now winterized? Can you post a pic of the whole pool?
You might want to reach out to a poster that goes by the name OnBalance. He has a company that researches problems in finish applications and may have much deeper insight to your photos than I.
I can sometimes assist with stain treatment as I'm on well and have fooled around/experimented and read a good deal with stains but this one may be a bit beyond me.
Sorry this is happening to you and I hope that you're able to get things put right!