Black Particulate upon Shocking

[MstrdAlgaeSucks]

HIya,

I have managed my in ground pool for 11 years, and we've had chronic mustard algea since season 2. I've gotten pretty good at managing it (copper-based algecide, loooots of shock, balanced chemistry, pretend phosphates don't exist cuz we have lost that battle, and we peg every reading ever taken on it).
This year, in addition to getting "taken" by a pool service guy who was..not good...and a chlorinator that was installed backwards, and yaddah yaddah...
the last 2 times I've shocked my pool (concentrated dry shock, dissolved in an 8 gallon bucket of water, then cast, while the pump is off because the filter keeps plugging), i get fine, dark (seems black, but when i pick it up it smears brownish on my skin) on the surface and the bottom of the pool, where i cast the shock. So there's a dark stripe on the floor wherever i cast, as if i had tossed metal shavings mixed in with the shock.

I've checked the dry shock - it's clean.
I've check the bucket - it's clean.
I use metal scale-out. our well water has iron algae in it. this occured last week when i had not put any well water into the pool, and again last night after i *had* put well water in. The result was the same. i am not getting the reddish-brown cast to the water and staining on the water line that i have in the past when i failed to the metals out of the water.
After a few hours, the dark particulate goes away. There is currently a LOT of light-colored particulate on the bottom that i will vacuum to waste once the water holds a chlorine reading for 4 to 8 hours after a shock and side-scrub. this isn't unusual, except that it looks light blue. i'm baffled by that. i have no idea what the pool guy i hired did before i realized he was unqualified and fired him.
My pool chemistry is pretty solid - stabilizer is low right now, but i can't bring it up until i can turn my pump back on once the algae is dead. pH and alkalinity are both in range.

suggestions?

 

[Texas Splash]

Welcome to TFP! A couple things come to my mind in your scenario:
1 - You have well water. We know the iron can react to the chlorine. I can't help but wonder if what you are seeing is a real-time effect of those chlorine granules reacting to the iron itself. For that I would ask @JoyfulNoise for his thoughts.
2 - We have to ask ... how are you testing the water? Can you provide a full set of test results from a TF-100/Pro-Series test kit or a Taylor K-2006C test kit. That would be an ideal place to start.

 

[MstrdAlgaeSucks]

Yeah...I was afraid you'd ask me that. I'm using standard test strips. I know i'm supposed to use a full kit, but when I originally tried to find one, and asked at local pool supply stores on how to properly test, I was handed a bottle of the simple test strips and told to use those.
So I use those.
as for this being a real-time reaction of the chlorine reacting with the iron...possible I suppose. But i hadn't put any well water into it before the first event. It was all rainwater and last year's water, which had already been treated for metals.

 

[MstrdAlgaeSucks]

Yeah...I was afraid you'd ask me that. I'm using standard test strips. I know i'm supposed to use a full kit, but when I originally tried to find one, and asked at local pool supply stores on how to properly test, I was handed a bottle of the simple test strips and told to use those.
So I use those.
as for this being a real-time reaction of the chlorine reacting with the iron...possible I suppose. But i hadn't put any well water into it before the first event. It was all rainwater and last year's water, which had already been treated for metals.

Oh, And i'm not a new member, technically. I first posted back in 2016, but I haven't been on in years.