Welcome, coach...hope you're planning to get your own TF-100 to manage all this going forward
In the mean time, also lower your ph to 7.2, which will help keep your metals in solution. That's something you can do right now.
The black stain could be copper, especially since you're using an algaecide with copper, its possible your copper levels have gotten too high, in which case dilution is best. Did you confirm it wasn't organic? Did the pool store test show copper? (Dilution is usually best for copper because its rare to find much copper in source water...its usually added by product, mineral system, or corrosion of copper pipes or heat exchanger.)
Don't use metal free again, its not really the kind of sequestrant we recommend here...for SWG use Jacks Magic Purple specifically after you dilute, and do not shock/slam after using it because so-doing breaks the sequestrant down.
Later on, to treat the black stain, provided it tests positive to acid (or Jacks Stain Test kit) and actually is copper, you might want to consider trying this tool for a "spot" acid wash:
Amazon.com: Purity Kit Underwater Acid Wash UAW: Home Audio Theater
If its copper staining, that will work best.
Lastly, this may be obvious to you by now, but no more pool store mixed elixirs or haphazzard guessing...that powder was adding copper. If you need to boost FC, use unscented bleach or pool store liquid chlorine only, and test your own water (
http://www.tftestkits.com) use the [fc/cya][/FC/cya] chart's swg setting to dial in your water.
If you weren't really using much in the way of Algaecides with copper and if you pool store test showed copper (metals are one thing their testing can generally be accurate on) you might also want to check your heat exchanger if you have a heater for corrosion in the event that is leaching copper into your water.
Cheers to clear!